code: private function checkDownloads()
      {
          // If images are to be shown, get out.
          if ($this->getShowImages()) {
              return;
          }
          if (($file = Input::get('file')) && ($key = Input::get('fileKey'))) {
              // Check key and return 403 if mismatch.
              if (!(array_key_exists($file, $_SESSION['metaModels_downloads'])
                  && $_SESSION['metaModels_downloads'][$file] === $key)) {
                  $objHandler = new $GLOBALS['TL_PTY']['error_403']();
                  /** @var PageError403 $objHandler */
                  $objHandler->generate($file);
              }
              // Send the file to the browser if check succeeded.
              Controller::sendFileToBrowser($file);
          }
      }
docs: Check if a file download is desired.
  
  See https://github.com/MetaModels/attribute_file/issues/6
  See https://github.com/MetaModels/core/issues/1014
  
  @return void
  
  @SuppressWarnings(PHPMD.Superglobals)
  @SuppressWarnings(PHPMD.CamelCaseVariableName)