I'm no help I'm afraid!
I just started using DFP 2, (I previously used the David Powers PHP solutions book to create my own, as before DFP 2, there was no facility for thumbnail creation.)
I think I am experiencing a similar problem.
I went through the wizard and have struggled with the document 'Adding Digital File Pro to DataAssist Insert and Update Pages'.
On my update page, i can get the default value to populate the text field and text areas, but I can't seem to get anything into the file field.
if i leave my original text field asis (not hidden) i can see the value displayed.
the upshot of this, is that when i update the record, the image field in my database is blank, unless i use the file field to reload it.
Perhaps it's me, but I don't find the 'solution' very clear.