process:
  fid: fid
  filename: filename
  uuid: uuid
  status: status
  created: timestamp
  changed: timestamp
  uid: uid
  [...]

/**
   * {@inheritdoc}
   */
  public function fields() {
    return array(
      'fid' => $this->t('File ID'),
      'uid' => $this->t('The {users}.uid who added the file. If set to 0, this file was added by an anonymous user.'),
      'filename' => $this->t('File name'),
      'filepath' => $this->t('File path'),
      'filemime' => $this->t('File MIME Type'),
      'status' => $this->t('The published status of a file.'),
      'timestamp' => $this->t('The time that the file was added.'),
      'uuid' => $this->t('UUID'),
    );
  }

$query = $this->select('file_managed', 'f')
      ->fields('f')
      ->orderBy('f.timestamp');