Ahhh, now I see the problem.
A file field cannot be assigned a value.
The basic idea is to create hidden form element, the initial value for the hidden element comes from the recordset. In the DFP Upload behavior, bind the default file option to the hidden form element. If no file is uploaded, it will use the default file value.
See the "How to apply Digital File Pro to DataAssist Insert and Update pages" tutorial on the How to section of the DFP Support page for more details: