Previews of private files uploaded by anonymous users don't work in Drupal 7.56

This module has a small regression introduced by the Drupal 7.56 security release: If an anonymous user uploads a private file using this module's widget, they can't see a preview of the file before it is submitted. See https://www.drupal.org/project/drupal/releases/7.56.

The ideal fix would be to change this module's custom file-saving function so it's a wrapper around file_save_upload() rather than a replacement for it, but I'm not sure offhand if that is possible. In the meantime, it's possible to just borrow the new code from core instead.