PHPSessionHandler: Implement SessionHandlerInterface

[mediawiki.git] / includes / media / MediaHandler.php

<?php
/**
 * Media-handling base classes and generic functionality.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup Media
 */

/**
 * Base media handler class
 *
 * @ingroup Media
 */
abstract class MediaHandler {
        const TRANSFORM_LATER = 1;
        const METADATA_GOOD = true;
        const METADATA_BAD = false;
        const METADATA_COMPATIBLE = 2; // for old but backwards compatible.
        /**
         * Max length of error logged by logErrorForExternalProcess()
         */
        const MAX_ERR_LOG_SIZE = 65535;

        /** @var MediaHandler[] Instance cache with array of MediaHandler */
        protected static $handlers = array();

        /**
         * Get a MediaHandler for a given MIME type from the instance cache
         *
         * @param string $type
         * @return MediaHandler
         */
        static function getHandler( $type ) {
                global $wgMediaHandlers;
                if ( !isset( $wgMediaHandlers[$type] ) ) {
                        wfDebug( __METHOD__ . ": no handler found for $type.\n" );

                        return false;
                }
                $class = $wgMediaHandlers[$type];
                if ( !isset( self::$handlers[$class] ) ) {
                        self::$handlers[$class] = new $class;
                        if ( !self::$handlers[$class]->isEnabled() ) {
                                wfDebug( __METHOD__ . ": $class is not enabled\n" );
                                self::$handlers[$class] = false;
                        }
                }

                return self::$handlers[$class];
        }

        /**
         * Resets all static caches
         */
        public static function resetCache() {
                self::$handlers = array();
        }

        /**
         * Get an associative array mapping magic word IDs to parameter names.
         * Will be used by the parser to identify parameters.
         */
        abstract function getParamMap();

        /**
         * Validate a thumbnail parameter at parse time.
         * Return true to accept the parameter, and false to reject it.
         * If you return false, the parser will do something quiet and forgiving.
         *
         * @param string $name
         * @param mixed $value
         */
        abstract function validateParam( $name, $value );

        /**
         * Merge a parameter array into a string appropriate for inclusion in filenames
         *
         * @param array $params Array of parameters that have been through normaliseParams.
         * @return string
         */
        abstract function makeParamString( $params );

        /**
         * Parse a param string made with makeParamString back into an array
         *
         * @param string $str The parameter string without file name (e.g. 122px)
         * @return array|bool Array of parameters or false on failure.
         */
        abstract function parseParamString( $str );

        /**
         * Changes the parameter array as necessary, ready for transformation.
         * Should be idempotent.
         * Returns false if the parameters are unacceptable and the transform should fail
         * @param File $image
         * @param array $params
         */
        abstract function normaliseParams( $image, &$params );

        /**
         * Get an image size array like that returned by getimagesize(), or false if it
         * can't be determined.
         *
         * This function is used for determining the width, height and bitdepth directly
         * from an image. The results are stored in the database in the img_width,
         * img_height, img_bits fields.
         *
         * @note If this is a multipage file, return the width and height of the
         *  first page.
         *
         * @param File $image The image object, or false if there isn't one
         * @param string $path The filename
         * @return array Follow the format of PHP getimagesize() internal function.
         *   See http://www.php.net/getimagesize. MediaWiki will only ever use the
         *   first two array keys (the width and height), and the 'bits' associative
         *   key. All other array keys are ignored. Returning a 'bits' key is optional
         *   as not all formats have a notion of "bitdepth".
         */
        abstract function getImageSize( $image, $path );

        /**
         * Get handler-specific metadata which will be saved in the img_metadata field.
         *
         * @param File $image The image object, or false if there isn't one.
         *   Warning, FSFile::getPropsFromPath might pass an (object)array() instead (!)
         * @param string $path The filename
         * @return string A string of metadata in php serialized form (Run through serialize())
         */
        function getMetadata( $image, $path ) {
                return '';
        }

        /**
         * Get metadata version.
         *
         * This is not used for validating metadata, this is used for the api when returning
         * metadata, since api content formats should stay the same over time, and so things
         * using ForeignApiRepo can keep backwards compatibility
         *
         * All core media handlers share a common version number, and extensions can
         * use the GetMetadataVersion hook to append to the array (they should append a unique
         * string so not to get confusing). If there was a media handler named 'foo' with metadata
         * version 3 it might add to the end of the array the element 'foo=3'. if the core metadata
         * version is 2, the end version string would look like '2;foo=3'.
         *
         * @return string Version string
         */
        static function getMetadataVersion() {
                $version = array( '2' ); // core metadata version
                Hooks::run( 'GetMetadataVersion', array( &$version ) );

                return implode( ';', $version );
        }

        /**
         * Convert metadata version.
         *
         * By default just returns $metadata, but can be used to allow
         * media handlers to convert between metadata versions.
         *
         * @param string|array $metadata Metadata array (serialized if string)
         * @param int $version Target version
         * @return array Serialized metadata in specified version, or $metadata on fail.
         */
        function convertMetadataVersion( $metadata, $version = 1 ) {
                if ( !is_array( $metadata ) ) {

                        // unserialize to keep return parameter consistent.
                        MediaWiki\suppressWarnings();
                        $ret = unserialize( $metadata );
                        MediaWiki\restoreWarnings();

                        return $ret;
                }

                return $metadata;
        }

        /**
         * Get a string describing the type of metadata, for display purposes.
         *
         * @note This method is currently unused.
         * @param File $image
         * @return string
         */
        function getMetadataType( $image ) {
                return false;
        }

        /**
         * Check if the metadata string is valid for this handler.
         * If it returns MediaHandler::METADATA_BAD (or false), Image
         * will reload the metadata from the file and update the database.
         * MediaHandler::METADATA_GOOD for if the metadata is a-ok,
         * MediaHandler::METADATA_COMPATIBLE if metadata is old but backwards
         * compatible (which may or may not trigger a metadata reload).
         *
         * @note Returning self::METADATA_BAD will trigger a metadata reload from
         *  file on page view. Always returning this from a broken file, or suddenly
         *  triggering as bad metadata for a large number of files can cause
         *  performance problems.
         * @param File $image
         * @param string $metadata The metadata in serialized form
         * @return bool
         */
        function isMetadataValid( $image, $metadata ) {
                return self::METADATA_GOOD;
        }

        /**
         * Get an array of standard (FormatMetadata type) metadata values.
         *
         * The returned data is largely the same as that from getMetadata(),
         * but formatted in a standard, stable, handler-independent way.
         * The idea being that some values like ImageDescription or Artist
         * are universal and should be retrievable in a handler generic way.
         *
         * The specific properties are the type of properties that can be
         * handled by the FormatMetadata class. These values are exposed to the
         * user via the filemetadata parser function.
         *
         * Details of the response format of this function can be found at
         * https://www.mediawiki.org/wiki/Manual:File_metadata_handling
         * tl/dr: the response is an associative array of
         * properties keyed by name, but the value can be complex. You probably
         * want to call one of the FormatMetadata::flatten* functions on the
         * property values before using them, or call
         * FormatMetadata::getFormattedData() on the full response array, which
         * transforms all values into prettified, human-readable text.
         *
         * Subclasses overriding this function must return a value which is a
         * valid API response fragment (all associative array keys are valid
         * XML tagnames).
         *
         * Note, if the file simply has no metadata, but the handler supports
         * this interface, it should return an empty array, not false.
         *
         * @param File $file
         * @return array|bool False if interface not supported
         * @since 1.23
         */
        public function getCommonMetaArray( File $file ) {
                return false;
        }

        /**
         * Get a MediaTransformOutput object representing an alternate of the transformed
         * output which will call an intermediary thumbnail assist script.
         *
         * Used when the repository has a thumbnailScriptUrl option configured.
         *
         * Return false to fall back to the regular getTransform().
         * @param File $image
         * @param string $script
         * @param array $params
         * @return bool|ThumbnailImage
         */
        function getScriptedTransform( $image, $script, $params ) {
                return false;
        }

        /**
         * Get a MediaTransformOutput object representing the transformed output. Does not
         * actually do the transform.
         *
         * @param File $image The image object
         * @param string $dstPath Filesystem destination path
         * @param string $dstUrl Destination URL to use in output HTML
         * @param array $params Arbitrary set of parameters validated by $this->validateParam()
         * @return MediaTransformOutput
         */
        final function getTransform( $image, $dstPath, $dstUrl, $params ) {
                return $this->doTransform( $image, $dstPath, $dstUrl, $params, self::TRANSFORM_LATER );
        }

        /**
         * Get a MediaTransformOutput object representing the transformed output. Does the
         * transform unless $flags contains self::TRANSFORM_LATER.
         *
         * @param File $image The image object
         * @param string $dstPath Filesystem destination path
         * @param string $dstUrl Destination URL to use in output HTML
         * @param array $params Arbitrary set of parameters validated by $this->validateParam()
         *   Note: These parameters have *not* gone through $this->normaliseParams()
         * @param int $flags A bitfield, may contain self::TRANSFORM_LATER
         * @return MediaTransformOutput
         */
        abstract function doTransform( $image, $dstPath, $dstUrl, $params, $flags = 0 );

        /**
         * Get the thumbnail extension and MIME type for a given source MIME type
         *
         * @param string $ext Extension of original file
         * @param string $mime MIME type of original file
         * @param array $params Handler specific rendering parameters
         * @return array Thumbnail extension and MIME type
         */
        function getThumbType( $ext, $mime, $params = null ) {
                $magic = MimeMagic::singleton();
                if ( !$ext || $magic->isMatchingExtension( $ext, $mime ) === false ) {
                        // The extension is not valid for this MIME type and we do
                        // recognize the MIME type
                        $extensions = $magic->getExtensionsForType( $mime );
                        if ( $extensions ) {
                                return array( strtok( $extensions, ' ' ), $mime );
                        }
                }

                // The extension is correct (true) or the MIME type is unknown to
                // MediaWiki (null)
                return array( $ext, $mime );
        }

        /**
         * Get useful response headers for GET/HEAD requests for a file with the given metadata
         *
         * @param mixed $metadata Result of the getMetadata() function of this handler for a file
         * @return array
         */
        public function getStreamHeaders( $metadata ) {
                return array();
        }

        /**
         * True if the handled types can be transformed
         *
         * @param File $file
         * @return bool
         */
        function canRender( $file ) {
                return true;
        }

        /**
         * True if handled types cannot be displayed directly in a browser
         * but can be rendered
         *
         * @param File $file
         * @return bool
         */
        function mustRender( $file ) {
                return false;
        }

        /**
         * True if the type has multi-page capabilities
         *
         * @param File $file
         * @return bool
         */
        function isMultiPage( $file ) {
                return false;
        }

        /**
         * Page count for a multi-page document, false if unsupported or unknown
         *
         * @param File $file
         * @return bool
         */
        function pageCount( File $file ) {
                return false;
        }

        /**
         * The material is vectorized and thus scaling is lossless
         *
         * @param File $file
         * @return bool
         */
        function isVectorized( $file ) {
                return false;
        }

        /**
         * The material is an image, and is animated.
         * In particular, video material need not return true.
         * @note Before 1.20, this was a method of ImageHandler only
         *
         * @param File $file
         * @return bool
         */
        function isAnimatedImage( $file ) {
                return false;
        }

        /**
         * If the material is animated, we can animate the thumbnail
         * @since 1.20
         *
         * @param File $file
         * @return bool If material is not animated, handler may return any value.
         */
        function canAnimateThumbnail( $file ) {
                return true;
        }

        /**
         * False if the handler is disabled for all files
         * @return bool
         */
        function isEnabled() {
                return true;
        }

        /**
         * Get an associative array of page dimensions
         * Currently "width" and "height" are understood, but this might be
         * expanded in the future.
         * Returns false if unknown.
         *
         * It is expected that handlers for paged media (e.g. DjVuHandler)
         * will override this method so that it gives the correct results
         * for each specific page of the file, using the $page argument.
         *
         * @note For non-paged media, use getImageSize.
         *
         * @param File $image
         * @param int $page What page to get dimensions of
         * @return array|bool
         */
        function getPageDimensions( File $image, $page ) {
                $gis = $this->getImageSize( $image, $image->getLocalRefPath() );
                if ( $gis ) {
                        return array(
                                'width' => $gis[0],
                                'height' => $gis[1]
                        );
                } else {
                        return false;
                }
        }

        /**
         * Generic getter for text layer.
         * Currently overloaded by PDF and DjVu handlers
         * @param File $image
         * @param int $page Page number to get information for
         * @return bool|string Page text or false when no text found or if
         *   unsupported.
         */
        function getPageText( File $image, $page ) {
                return false;
        }

        /**
         * Get the text of the entire document.
         * @param File $file
         * @return bool|string The text of the document or false if unsupported.
         */
        public function getEntireText( File $file ) {
                $numPages = $file->pageCount();
                if ( !$numPages ) {
                        // Not a multipage document
                        return $this->getPageText( $file, 1 );
                }
                $document = '';
                for ( $i = 1; $i <= $numPages; $i++ ) {
                        $curPage = $this->getPageText( $file, $i );
                        if ( is_string( $curPage ) ) {
                                $document .= $curPage . "\n";
                        }
                }
                if ( $document !== '' ) {
                        return $document;
                }
                return false;
        }

        /**
         * Get an array structure that looks like this:
         *
         * array(
         *    'visible' => array(
         *       'Human-readable name' => 'Human readable value',
         *       ...
         *    ),
         *    'collapsed' => array(
         *       'Human-readable name' => 'Human readable value',
         *       ...
         *    )
         * )
         * The UI will format this into a table where the visible fields are always
         * visible, and the collapsed fields are optionally visible.
         *
         * The function should return false if there is no metadata to display.
         */

        /**
         * @todo FIXME: This interface is not very flexible. The media handler
         * should generate HTML instead. It can do all the formatting according
         * to some standard. That makes it possible to do things like visual
         * indication of grouped and chained streams in ogg container files.
         * @param File $image
         * @param bool|IContextSource $context Context to use (optional)
         * @return array|bool
         */
        function formatMetadata( $image, $context = false ) {
                return false;
        }

        /** sorts the visible/invisible field.
         * Split off from ImageHandler::formatMetadata, as used by more than
         * one type of handler.
         *
         * This is used by the media handlers that use the FormatMetadata class
         *
         * @param array $metadataArray Metadata array
         * @param bool|IContextSource $context Context to use (optional)
         * @return array Array for use displaying metadata.
         */
        function formatMetadataHelper( $metadataArray, $context = false ) {
                $result = array(
                        'visible' => array(),
                        'collapsed' => array()
                );

                $formatted = FormatMetadata::getFormattedData( $metadataArray, $context );
                // Sort fields into visible and collapsed
                $visibleFields = $this->visibleMetadataFields();
                foreach ( $formatted as $name => $value ) {
                        $tag = strtolower( $name );
                        self::addMeta( $result,
                                in_array( $tag, $visibleFields ) ? 'visible' : 'collapsed',
                                'exif',
                                $tag,
                                $value
                        );
                }

                return $result;
        }

        /**
         * Get a list of metadata items which should be displayed when
         * the metadata table is collapsed.
         *
         * @return array Array of strings
         */
        protected function visibleMetadataFields() {
                return FormatMetadata::getVisibleFields();
        }

        /**
         * This is used to generate an array element for each metadata value
         * That array is then used to generate the table of metadata values
         * on the image page
         *
         * @param array &$array An array containing elements for each type of visibility
         *   and each of those elements being an array of metadata items. This function adds
         *   a value to that array.
         * @param string $visibility ('visible' or 'collapsed') if this value is hidden
         *   by default.
         * @param string $type Type of metadata tag (currently always 'exif')
         * @param string $id The name of the metadata tag (like 'artist' for example).
         *   its name in the table displayed is the message "$type-$id" (Ex exif-artist ).
         * @param string $value Thingy goes into a wikitext table; it used to be escaped but
         *   that was incompatible with previous practise of customized display
         *   with wikitext formatting via messages such as 'exif-model-value'.
         *   So the escaping is taken back out, but generally this seems a confusing
         *   interface.
         * @param bool|string $param Value to pass to the message for the name of the field
         *   as $1. Currently this parameter doesn't seem to ever be used.
         *
         * Note, everything here is passed through the parser later on (!)
         */
        protected static function addMeta( &$array, $visibility, $type, $id, $value, $param = false ) {
                $msg = wfMessage( "$type-$id", $param );
                if ( $msg->exists() ) {
                        $name = $msg->text();
                } else {
                        // This is for future compatibility when using instant commons.
                        // So as to not display as ugly a name if a new metadata
                        // property is defined that we don't know about
                        // (not a major issue since such a property would be collapsed
                        // by default).
                        wfDebug( __METHOD__ . ' Unknown metadata name: ' . $id . "\n" );
                        $name = wfEscapeWikiText( $id );
                }
                $array[$visibility][] = array(
                        'id' => "$type-$id",
                        'name' => $name,
                        'value' => $value
                );
        }

        /**
         * Short description. Shown on Special:Search results.
         *
         * @param File $file
         * @return string
         */
        function getShortDesc( $file ) {
                return self::getGeneralShortDesc( $file );
        }

        /**
         * Long description. Shown under image on image description page surounded by ().
         *
         * @param File $file
         * @return string
         */
        function getLongDesc( $file ) {
                return self::getGeneralLongDesc( $file );
        }

        /**
         * Used instead of getShortDesc if there is no handler registered for file.
         *
         * @param File $file
         * @return string
         */
        static function getGeneralShortDesc( $file ) {
                global $wgLang;

                return htmlspecialchars( $wgLang->formatSize( $file->getSize() ) );
        }

        /**
         * Used instead of getLongDesc if there is no handler registered for file.
         *
         * @param File $file
         * @return string
         */
        static function getGeneralLongDesc( $file ) {
                return wfMessage( 'file-info' )->sizeParams( $file->getSize() )
                        ->params( '<span class="mime-type">' . $file->getMimeType() . '</span>' )->parse();
        }

        /**
         * Calculate the largest thumbnail width for a given original file size
         * such that the thumbnail's height is at most $maxHeight.
         * @param int $boxWidth Width of the thumbnail box.
         * @param int $boxHeight Height of the thumbnail box.
         * @param int $maxHeight Maximum height expected for the thumbnail.
         * @return int
         */
        public static function fitBoxWidth( $boxWidth, $boxHeight, $maxHeight ) {
                $idealWidth = $boxWidth * $maxHeight / $boxHeight;
                $roundedUp = ceil( $idealWidth );
                if ( round( $roundedUp * $boxHeight / $boxWidth ) > $maxHeight ) {
                        return floor( $idealWidth );
                } else {
                        return $roundedUp;
                }
        }

        /**
         * Shown in file history box on image description page.
         *
         * @param File $file
         * @return string Dimensions
         */
        function getDimensionsString( $file ) {
                return '';
        }

        /**
         * Modify the parser object post-transform.
         *
         * This is often used to do $parser->addOutputHook(),
         * in order to add some javascript to render a viewer.
         * See TimedMediaHandler or OggHandler for an example.
         *
         * @param Parser $parser
         * @param File $file
         */
        function parserTransformHook( $parser, $file ) {
        }

        /**
         * File validation hook called on upload.
         *
         * If the file at the given local path is not valid, or its MIME type does not
         * match the handler class, a Status object should be returned containing
         * relevant errors.
         *
         * @param string $fileName The local path to the file.
         * @return Status
         */
        function verifyUpload( $fileName ) {
                return Status::newGood();
        }

        /**
         * Check for zero-sized thumbnails. These can be generated when
         * no disk space is available or some other error occurs
         *
         * @param string $dstPath The location of the suspect file
         * @param int $retval Return value of some shell process, file will be deleted if this is non-zero
         * @return bool True if removed, false otherwise
         */
        function removeBadFile( $dstPath, $retval = 0 ) {
                if ( file_exists( $dstPath ) ) {
                        $thumbstat = stat( $dstPath );
                        if ( $thumbstat['size'] == 0 || $retval != 0 ) {
                                $result = unlink( $dstPath );

                                if ( $result ) {
                                        wfDebugLog( 'thumbnail',
                                                sprintf( 'Removing bad %d-byte thumbnail "%s". unlink() succeeded',
                                                        $thumbstat['size'], $dstPath ) );
                                } else {
                                        wfDebugLog( 'thumbnail',
                                                sprintf( 'Removing bad %d-byte thumbnail "%s". unlink() failed',
                                                        $thumbstat['size'], $dstPath ) );
                                }

                                return true;
                        }
                }

                return false;
        }

        /**
         * Remove files from the purge list.
         *
         * This is used by some video handlers to prevent ?action=purge
         * from removing a transcoded video, which is expensive to
         * regenerate.
         *
         * @see LocalFile::purgeThumbnails
         *
         * @param array $files
         * @param array $options Purge options. Currently will always be
         *  an array with a single key 'forThumbRefresh' set to true.
         */
        public function filterThumbnailPurgeList( &$files, $options ) {
                // Do nothing
        }

        /**
         * True if the handler can rotate the media
         * @since 1.24 non-static. From 1.21-1.23 was static
         * @return bool
         */
        public function canRotate() {
                return false;
        }

        /**
         * On supporting image formats, try to read out the low-level orientation
         * of the file and return the angle that the file needs to be rotated to
         * be viewed.
         *
         * This information is only useful when manipulating the original file;
         * the width and height we normally work with is logical, and will match
         * any produced output views.
         *
         * For files we don't know, we return 0.
         *
         * @param File $file
         * @return int 0, 90, 180 or 270
         */
        public function getRotation( $file ) {
                return 0;
        }

        /**
         * Log an error that occurred in an external process
         *
         * Moved from BitmapHandler to MediaHandler with MediaWiki 1.23
         *
         * @since 1.23
         * @param int $retval
         * @param string $err Error reported by command. Anything longer than
         * MediaHandler::MAX_ERR_LOG_SIZE is stripped off.
         * @param string $cmd
         */
        protected function logErrorForExternalProcess( $retval, $err, $cmd ) {
                # Keep error output limited (bug 57985)
                $errMessage = trim( substr( $err, 0, self::MAX_ERR_LOG_SIZE ) );

                wfDebugLog( 'thumbnail',
                        sprintf( 'thumbnail failed on %s: error %d "%s" from "%s"',
                                        wfHostname(), $retval, $errMessage, $cmd ) );
        }

        /**
         * Get list of languages file can be viewed in.
         *
         * @param File $file
         * @return string[] Array of language codes, or empty array if unsupported.
         * @since 1.23
         */
        public function getAvailableLanguages( File $file ) {
                return array();
        }

        /**
         * On file types that support renderings in multiple languages,
         * which language is used by default if unspecified.
         *
         * If getAvailableLanguages returns a non-empty array, this must return
         * a valid language code. Otherwise can return null if files of this
         * type do not support alternative language renderings.
         *
         * @param File $file
         * @return string|null Language code or null if multi-language not supported for filetype.
         * @since 1.23
         */
        public function getDefaultRenderLanguage( File $file ) {
                return null;
        }

        /**
         * If its an audio file, return the length of the file. Otherwise 0.
         *
         * File::getLength() existed for a long time, but was calling a method
         * that only existed in some subclasses of this class (The TMH ones).
         *
         * @param File $file
         * @return float Length in seconds
         * @since 1.23
         */
        public function getLength( $file ) {
                return 0.0;
        }

        /**
         * True if creating thumbnails from the file is large or otherwise resource-intensive.
         * @param File $file
         * @return bool
         */
        public function isExpensiveToThumbnail( $file ) {
                return false;
        }

        /**
         * Returns whether or not this handler supports the chained generation of thumbnails according
         * to buckets
         * @return bool
         * @since 1.24
         */
        public function supportsBucketing() {
                return false;
        }

        /**
         * Returns a normalised params array for which parameters have been cleaned up for bucketing
         * purposes
         * @param array $params
         * @return array
         */
        public function sanitizeParamsForBucketing( $params ) {
                return $params;
        }

        /**
         * Gets configuration for the file warning message. Return value of
         * the following structure:
         *   array(
         *     // Required, module with messages loaded for the client
         *     'module' => 'example.filewarning.messages',
         *     // Required, array of names of messages
         *     'messages' => array(
         *       // Required, main warning message
         *       'main' => 'example-filewarning-main',
         *       // Optional, header for warning dialog
         *       'header' => 'example-filewarning-header',
         *       // Optional, footer for warning dialog
         *       'footer' => 'example-filewarning-footer',
         *       // Optional, text for more-information link (see below)
         *       'info' => 'example-filewarning-info',
         *     ),
         *     // Optional, link for more information
         *     'link' => 'http://example.com',
         *   )
         *
         * Returns null if no warning is necessary.
         * @param File $file
         * @return array|null
         */
        public function getWarningConfig( $file ) {
                return null;
        }
}