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

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