Allow WordPress to upload MP4, WebM and OGG video

additional-mime-types

add_filter( 'mime_types', 'aco_extend_mime_types' );

function aco_extend_mime_types( $existing_mimes ) {

  // Add webm, mp4 and OGG to the list of mime types
  $existing_mimes['webm'] = 'video/webm';
  $existing_mimes['mp4']  = 'video/mp4';
  $existing_mimes['ogg']  = 'video/ogg';

  // Return an array now including our added mime types
  return $existing_mimes;
}