I think it could be very usefull be able to add an attachment in the signature form page.
Lots of people put pictures into their signature. Actually, there is no way to upload an image from the signature form page. Therefore, users have to upload their pictures somewhere else or go to the "create a new post" form page and create an attachments.
I imagine you didn't add this function in this form page because users could send text or archives. Perhaps could you just allow to upload images (like .png, .jpg, .jpeg, .gif format).