Oh that's really annoying, sorry neither of those tips worked.

I think the problem is that current doesn't yet know what the record's sys ID will be. Typically a current object's sys ID will be represented as -1 until the record is saved.

I will defer to others in this case as I can't think of a way to do this without some hefty customisation. Although you might consider just allowing people to attach files at the top-level record producer and not associating them with each row in the variable set, since there'll be some element of human interaction on the case to validate the attachments and set up the individual contracts.