class ContentNegotiation

ExceptionControllerTest.php in drupal/core/modules/system/lib/Drupal/system/Tests/System/ExceptionControllerTest.php

ViewSubscriber.php in drupal/core/lib/Drupal/Core/EventSubscriber/ViewSubscriber.php

Definition of Drupal\Core\EventSubscriber\ViewSubscriber.

class ContentNegotiation {

  /**
   * Gets the normalized type of a request.
   *
   * The normalized type is a short, lowercase version of the format, such as
   * 'html', 'json' or 'atom'.
   *
   * @param Symfony\Component\HttpFoundation\Request $request
   *   The request object from which to extract the content type.
   *
   * @return
   *   The normalized type of a given request.
   */
  public function getContentType(Request $request) {

    // AJAX iframe uploads need special handling, because they contain a JSON
    // response wrapped in <textarea>.
    if ($request
      ->get('ajax_iframe_upload', FALSE)) {
      return 'iframeupload';
    }
    elseif ($request
      ->isXmlHttpRequest()) {
      return 'ajax';
    }

    // Check all formats, it HTML is found return it.
    $first_found_format = FALSE;
    foreach ($request
      ->getAcceptableContentTypes() as $mime_type) {
      $format = $request
        ->getFormat($mime_type);
      if ($format === 'html') {
        return $format;
      }
      if (!is_null($format) && !$first_found_format) {
        $first_found_format = $format;
      }
    }

    // No HTML found, return first found.
    if ($first_found_format) {
      return $first_found_format;
    }

    // Do HTML last so that it always wins.
    return 'html';
  }

}