/**
 * Implements hook_file_download_access_alter().
 */
function {{ machine_name }}_file_download_access_alter(&$grants, $file_item, $entity_type, $entity) {
  // For our example module, we always enforce the rules set by node module.
  if (isset($grants['node'])) {
    $grants = array('node' => $grants['node']);
  }
}