/**
 * Implements hook_file_move().
 */
function {{ machine_name }}_file_move(\Drupal\file\FileInterface $file, \Drupal\file\FileInterface $source) {
  // Make sure that the file name starts with the owner's user name.
  if (strpos($file->getFilename(), $file->getOwner()->name) !== 0) {
    $file->setFilename($file->getOwner()->name . '_' . $file->getFilename());
    $file->save();

    \Drupal::logger('file')->notice('Moved file %source has been renamed to %destination', ['%source' => $source->filename, '%destination' => $file->getFilename()]);
  }
}