code: private function getFiles(FileUpload $uploader): array
      {
          $files = $uploader->uploadTo('system/tmp');
  
          if (\count($files) < 1) {
              throw new \RuntimeException($this->translator->trans('ERR.all_fields', [], 'contao_default'));
          }
  
          foreach ($files as &$file) {
              $extension = pathinfo($file, PATHINFO_EXTENSION);
  
              if ('csv' !== $extension) {
                  throw new \RuntimeException(
                      sprintf($this->translator->trans('ERR.filetype', [], 'contao_default'), $extension)
                  );
              }
  
              $file = $this->projectDir.'/'.$file;
          }
  
          return $files;
      }
docs: Returns the uploaded files from a FileUpload instance.
  
  @throws \RuntimeException
  
  @return string[]