In your code, you'll see this line:
'DefaultFileName' => "".$row_WADAcontent['photo'] ."",
I am guessing that $row_WADAconente['photo'] is coming for your recordset. I am saying instead of populating in that matter, add a hidden field in your form that receives that value before updating, so in your form you will have something like this:
<input type="hidden" name="imagefile" value="<?php echo $row_WADAcontent['photo']; ?>">
Then, you will change that default file name value in your code to this:
'DefaultFileName' => "".$_POST['imagefile'] ."",
You will actually be able to set this value, as well, by double clicking the DFP server behavior and using the dialogue box on the second page... you will see the lightning bolt beside the default file field and from there you will want to select the form's hidden field.
That way, you default file will be generated at the same time as the form and then passed through as a form variable.