file method
Implementation
RequestFile? file(String key) {
  if (_all[key] == null) {
    return null;
  }
  if (_all[key] is! RequestFile) {
    return (_all[key] as List<RequestFile>).first;
  }
  return _all[key];
}RequestFile? file(String key) {
  if (_all[key] == null) {
    return null;
  }
  if (_all[key] is! RequestFile) {
    return (_all[key] as List<RequestFile>).first;
  }
  return _all[key];
}