Instead of using a separate Insert record Server behavior, you can edit the one that DataAssist adds to the page.
Also instead of using the simple file name from the form collection, Expand the Uploaded Files collection in the Dynamic Data Bindings and use the Server File name.
There is also a tutorial availabel to help with this available on the Universal Email support page:
universal-email/
look in the how to section for the how to titled "Applying File Upload to DataAssist Insert and Update pages"