/**
  * Extracts the files from a request.
  *
  * @param FacebookRequest $request
  *
  * @return string|null
  *
  * @throws FacebookSDKException
  */
 public function extractFileAttachments(FacebookRequest $request)
 {
     if (!$request->containsFileUploads()) {
         return null;
     }
     $files = $request->getFiles();
     $fileNames = [];
     foreach ($files as $file) {
         $fileName = uniqid();
         $this->addFile($fileName, $file);
         $fileNames[] = $fileName;
     }
     $request->resetFiles();
     // @TODO Does Graph support multiple uploads on one endpoint?
     return implode(',', $fileNames);
 }