You won't actually need to set the value in a hidden field... it is actually much easier than this... simply set up your DFP Upload by selecting the file field as you did... then go to either your "Insert Record" server behavior or your "Update Record" server behavior (depending on what kind of page it is).
Then double click to open the wizard... if your using DataAssist Insert Record, hit the "next" button in the dialogue box and then you'll see where you can assign values to each column as it corresponds in your database. Highlight the column that will hold your image or file name - you will then see the "value" input box below... you should also see a lightning bolt button which will allow you to set the dynamically generated value of your desire. Hit that button - then expand the WA Upload Files (please note there are two - the first one is global status - the second one, which is the one you want to expand, will have your input's id in parentheses behind it - that's the one you want). After you expand it, select "Server File Name" and then click OK to add that value to your column in the database.
Hope that helps,