Versioning files with a different file type

We often have projects that start out as Word docs and eventually become PDFs. Since we can't "stack" new versions of a file with a different file type from the original, we end up with both a Word doc & a PDF attached to the task. This can be problematic for us because often, additional edits are made during the PDF stage that aren't reflected in the Word doc. Our process is that once the Word doc content is put into layout, the doc is "dead" & the proofs from that point on are in PDF format. That way it's clear if we refer to the Wrike task for future edits that only the PDF should be referenced for the final version of the copy. 

This appears to be possible with JPG/PNG file types — I can upload a PNG as a new version of a previous JPG file. It would be great if we could also do so between Word & PDF.