A file field cannot be assigned a value.
since you cannot give a file field a value, you have to create a hidden form field to hold the file name from the database.
In the Digital File pro Wizard, you have to bind the Defulat File setting to the hidden form element.
When no file is uploaded, the DFP Server File name variable will use the Default File value.
Can you send a copy of your page so I can take a look at the code. also send a copy of your WA_DigitalFilePro/Helper_PHP.php file