There is not currently a way to dynamically change the emailed filename. We will look at this one more time to see if a solution is possible and get back to you.
We have added dynamic filename functionality to the Field Map Layout. You may now enter Formidable field shortcodes as well as static text into the “PDF file name in e-mails:” text box located just below the “Attach file to Email notifications” option on each field map layout.
Update your plugin to acquire the functionality. We remain at your service.