Really I would suggest getting DataAssist, which allows you to update a field with any value from any source not just directly from a form.
To use the DW Insert, you would:
1) add a hidden form element to the page
2) bind the update to the value of the hidden form element
3) in between the code that uploads the file and the code that inserts, add code:
<?php
$_POST['HiddenFormElementName'] = {server file name value from bindings};
?>