I found this in "digitalfilepro_da_insert_update_ht.pdf"
You are right, but I found this in the above mentioned helpfile. They suggest to manually insert a hidden field:
Applying Digital File Pro to your Update page
With your Insert page complete, you now need to do the same with the Update page. The process is similar, but requires an additional step.
Creating the file field
Use the following instructions for creating this field for your Insert page.
From the Dreamweaver Files panel, open your DataAssist Insert or Update page.
Select the field you want to replace with a file upload field.
On your keyboard, press Ctrl (Cmd) + T. This displays the Quick Tag Editor.
Change <input type=”text”> to <input type=”hidden”>.
Note: This is done to allow us to display the value for the form field that is currently entered into the database.
Place your cursor beside the hidden form field icon.
Choose the File field option from Forms category of the Insert bar.
Place your cursor in the ID field and enter an idea for this field.
Leave the Label field blank.