Custom thumbnail for each file type

You can set custom thumbnail for each file type of allowed media type. Use rtmedia_allowed_types filter in your theme or plugin and set ext_thumb value for that media type.

Following is the demo code to set custom thumbnail for .mp4 and .mov file format of media type video.

    add_filter( 'rtmedia_allowed_types', 'rtmedia_custom_thumb_mp4_mov', 999, 1 );

    function rtmedia_custom_thumb_mp4_mov( $types ){
        if( isset( $types['video'] ) ){
            $types['video']['ext_thumb'] = array(
                'mp4' => 'thumbnail url here',
                'wmv' => 'thumbnail url here',
        return $types;