public function MailFileDownload($mail_id, $file_id)
 {
     // $mail_id : IDType (only one)
     // $file_id : IDType (only one)
     $this->CheckAndSetHeader(__FUNCTION__);
     $reg_args["mail_id"] = $mail_id;
     $reg_args["file_id"] = $file_id;
     $this->encodeString($reg_args);
     $results = parent::MailFileDownload($reg_args);
     $this->methodClose();
     return $results->file->content;
 }