public static function add($file_url = '', $force_upload = false)
 {
     $uploader = new self();
     $uploader->file_url = $file_url;
     $already_uploaded = $uploader->attachment_exists();
     if ($already_uploaded && $force_upload === false) {
         return $already_uploaded;
     }
     $uploader->upload();
     if (!$uploader->file_data) {
         return false;
     }
     $uploader->add_to_media_library();
     return $uploader->attachment_id;
 }