code: public function onUpload(PostPersistEvent $event)
      {
          $file = $event->getFile();
          if (!$file instanceof FlysystemFile) {
              return;
          }
  
          $originalFilename = $file->getBasename();
          try {
              // Couldn't find anything better with OneUp uploader...
              $originalFiles = $event->getRequest()->files->all();
              if (isset($originalFiles['files'])) {
                  $originalFiles = $originalFiles['files'];
                  if (count($originalFiles)) {
                      $originalFile = array_pop($originalFiles);
                      $originalFilename = $originalFile->getClientOriginalName();
                  }
              }
          } catch (\Exception $e) {
          }
  
          $file = $this->resourceManager->addFile($file, $originalFilename, $event->getType());
  
          /** @var AbstractResponse $response */
          $response = $event->getResponse();
          $response[] = $file;
  
          return $response;
      }
docs: @param PostPersistEvent $event
  
  @throws \UnexpectedValueException
  @throws \InvalidArgumentException
  @throws \RuntimeException
  
  @return AbstractResponse