Removal of core embedding support for WMV and WMA file formats

With the introduction of the video widget (#39994) and audio widget (#39995) it was discovered that the WMV and WMA file formats are no longer supported by MediaElement.js as of v3. While core is currently shipping with MediaElement.js v2.22, these file formats only play if the Silverlight plugin is installed, something which is less and less common since the plugin is no longer supported as of Chrome 45 and as of Firefox 52. So with the planned upgrade of MediaElement.js from 2.22 to 4.x (see #39686), it is a good time with the WP 4.8 major release to remove support for the formats. This has been done in [40813] for #40819.

Plugins may continue to add embedding support for these file formats by re-adding them via the wp_video_extensions and wp_audio_extensions filters, for example:

function add_wmv_to_wp_video_extensions( $extensions ) {
    $extensions[] = 'wmv';
    return $extensions;
}
add_filter( 'wp_video_extensions', 'add_wmv_to_wp_video_extensions' );

function add_wma_to_wp_audio_extensions( $extensions ) {
    $extensions[] = 'wma';
    return $extensions;
}
add_filter( 'wp_audio_extensions', 'add_wma_to_wp_audio_extensions' );

Plugins would also need to implement fallback rendering routines via the wp_video_shortcode_override and wp_audio_shortcode_override filters, or else use a different library than MediaElement.js altogether via the wp_video_shortcode_library and wp_audio_shortcode_library filters.

Note that there is no functional difference before and after the removal of WMA and WMV support. When MediaElement.js doesn’t support a format, it displays a download link. Before and after [40813], the following is ho  the audio and video shortcodes look like with WMA and WMV files respectively:

In the admin, the functional difference is that these formats are not listed as being available for selection in the Video and Audio widgets. Additionally, when you Add Media in the post editor, there is now no option to embed the player for these formats; only a link to the media will be provided, just as when attempting to add non-media files to content.

#4-8, #core-media, #dev-notes, #mediaelement