Hi,
I fell foul of this as well and I couldn't find a workaround. Once the form is signed the process of attaching a file appears to be considered too much of a change.
For one form we dropped the digital signatures and went with a less secure/robust method of indicating that a section was signed, just so that we could maintain the ability of attaching files throughout the form process.
I'm afraid it is either one or the other - digital signature or attach files.
Good luck,
Niall