This is a bit of a quandary. You cant reliably know the ID that will be used for the record untill the insert has taken place.
I think to do this you will need to use a three step process:
In the data assist insert behavior, there is a setting called Store As, this stores the ID of the record that was inserted into a session variable.
You could insert the record with all of the other information from the form, use the Session Created by the Insert Record behavior in the File upload path, then Update the record to store the uploaded file name.
Make sure that the Insert record behavior does not have a redirect set. you will probably need to rearrange the code for the behaviors to get into the right order and you'll also want to use the Cookies Toolkit unset session behavior to clear the session after the update has taken place.