Move Separate Document into Another File's New Proof Version

[#1]

I've noticed you need to be the owner of the proof in order to upload a new proof version. We ran into an issue where our graphic designer uploaded a new document because they couldn't upload a new proof version, and users asked if the two files could be combined. Because they both were uploaded in the Documents tab I thought I could move that file as a new proof version of another file, but that isn't possible. It would be awesome if we could do that! We have multiple designers working on various projects and sometimes another one jumps in to finish the job.