The best way to do this is:
Inside your form with the update file field, create a hidden field beside it and store the name of the original file from the recordset in the hidden field.
Then in digital file pro, for the default file name, hit the lightning bolt and set it to the hidden form element.
Then, open the DataAssist server update behavior... be sure to set the column that holds your image name to the Digital File Pro server file name. Now, if you have a new file, it will use that... if no new file is added, it will revert to what you specified as default.
I typically use a naming convention like "image" for the hidden field and "imagefile" for the new file field.
Let me know if you have any issues,