Filter is use to restrict set custom thumbnail for any media type.
Note:: Here media type for album is album-edit.
add_filter( 'rtm_allow_custom_thumbnail', 'rtmedia_allow_custom_thumbnail', 10, 2 );
function rtmedia_allow_custom_thumbnail( $allow, $media_type ) {
if ( 'video' == $media_type ) {
$allow = false;
}
return $allow;
}