/**
 * Implements hook_file_update().
 */
function {{ machine_name }}_file_update($file) {
  $file_user = user_load($file->uid);
  // Make sure that the file name starts with the owner's user name.
  if (strpos($file->filename, $file_user->name) !== 0) {
    $old_filename = $file->filename;
    $file->filename = $file_user->name . '_' . $file->filename;
    $file->save();

    watchdog('file', t('%source has been renamed to %destination', array('%source' => $old_filename, '%destination' => $file->filename)));
  }
}