code: public function setFilesGroupsToUse($fileGroups) {
          if (empty($fileGroups)) {
              throw new \InvalidArgumentException('$fileGroups argument cannot be empty');
          } else if (!is_array($fileGroups) && !($fileGroups instanceof \Closure)) {
              throw new \InvalidArgumentException('$fileGroups argument must be an array or \Closure');
          }
          $this->fileConfigsToUse = $fileGroups;
          return $this;
      }
docs: List of file names to accept.
  Only provided files will be shown in form. Other files will be ignored (and won't be changed in any way)
  @param array|\Closure $fileGroups - \Closure must return array
  @return $this
  @throws \InvalidArgumentException