We've created forms to accept Excel files from our customers. The problem is the form will not accept an attachment with just the Attachment field on the form. We have to add a random field just to get the form to accept an attachment.
I just ran into this issue for the first time. I'm using hidden form fields using form field url queries (an awesome feature), so I only need the attachment field on the form. I'll have to add one to just to overcome this limitation.