Basically, you have a hidden form element on your page that is set to contain the file name that is already in the database.
in the Upload File server behavior, you set the Default File option to get the value from the hidden form element. This allows the database to be updated with the origianl file name if no file is uploaded.
The "How to apply Digital File Pro to DataAssist Insert and Update pages" tutorial in the How To section of the Digital File Pro support page explains how this is accomplished:
digital-file-pro/