Attach excel stream to swiftmailer message?

I'm trying to attach an Excel file in a SwiftMailer message. The trick is that I don't want to save the excel file and then attach it and then delete it, instead I just want to generate the excel and attach that to the message. This function allows to attach a OutputByteStream ``` /** * Create a new Attachment. * * @param string|Swift_OutputByteStream $data * @param string $filename * @param string $contentType * * @return Swift_Mime_Attachment */ public static function newInstance($data = null, $filename = null, $contentType = null) { return new self($data, $filename, $contentType); } ``` There is a function in Symfony PHPExcel bundle to create the response ``` /** * Stream the file as Response. * * @param \PHPExcel_Writer_IWriter $writer * @param int $status * @param array $headers * * @return StreamedResponse */ public function createStreamedResponse(\PHPExcel_Writer_IWriter $writer, $status = 200, $headers = array()) { return new StreamedResponse( function () use ($writer) { $writer->save('php://output'); }, $status, $headers ); } ``` They seems to call to that callback when they are rendering the reponse, but How can I save the php://output in a variable or something to pass it to newInstance method? I tried passing the response object (StreamedResponse) but it only have the headers, I also tried with $response->getContent() and passing $writer->save('php://output') to newInstance method.