upload p.o. as attachment

For some customers, it would be nice to setup an option for the customer to attach a physical P.O. in pdf format along with their order. I think a simple 'Attach PO' form that uploads to 'files/po_folder' would suffice. That way they don't have to fax or email it separately. If this could be a permission granted by user role, it would also be nice. (ie. some roles are not required to attach a PO while others are.)