Symfony\Component\HttpFoundation\Request::setRequestFormat PHP Method

    public function setRequestFormat($format)
    {
        $this->format = $format;
    }

 /**
  * {@inheritdoc}
  */
 public function handle(Request $request, $type = self::MASTER_REQUEST, $catch = TRUE)
 {
     if ($request->headers->has('Accept')) {
         $request->setRequestFormat($request->getFormat($request->headers->get('Accept')));
     }
     return $this->httpKernel->handle($request, $type, $catch);
 }