Hi Yoav,
Sorry for the delay in responding to you. This is a good question. My guess is that the digital signature will be calculated based on the file attachment since contents must not change and the file attachment is considered contents (and in fact base64-encoded in the form's data). You could try a little experiment and create a form with one and then try to delete the file attachment after signing (use notepad) and see if the form can still be opened without an error.
Patrick Halstead
Project Manager at Qdabra