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

    public function getMimeType($format)
    {
        if (null === static::$formats) {
            static::initializeFormats();
        }

        return isset(static::$formats[$format]) ? static::$formats[$format][0] : null;
    }

 protected function doKernelResponse(Request $request, Response $response)
 {
     if (!$response instanceof DataResponse) {
         return;
     }
     $routeName = $request->attributes->get('_route');
     $route = $this->routes->get($routeName);
     if (!$route) {
         return;
     }
     $acceptedFormat = $route->getOption(RouteOptions::ACCEPTED_FORMAT);
     if (!$acceptedFormat) {
         $response->setContent('');
         $response->setStatusCode(406);
     }
     if ($this->encoder->supportsEncoding($acceptedFormat) && $acceptedFormat === 'json') {
         $contentType = $request->getMimeType($acceptedFormat);
         $jsonResponse = new JsonResponse($response->getContent());
         $response->setContent($jsonResponse->getContent());
         $response->headers->set('Content-Type', $contentType);
     } elseif ($this->encoder->supportsEncoding($acceptedFormat)) {
         $contentType = $request->getMimeType($acceptedFormat);
         $content = $this->encoder->encode($response->getContent(), $acceptedFormat);
         $response->setContent($content);
         $response->headers->set('Content-Type', $contentType);
     }
 }