Has anyone ever had any luck with a CSV import that sets the value of a field this is a Document type? I tried to do it years ago, but could never figure it out. Hopefully something has changed. Do I just use the name of the file or does it have to be the full file path? It would be nice if you could use the internal ID, but that doesn’t seem to be an option.

I don't think you can. I vaguely recall working on an implementation requirement like this for a customer many years ago. IIRC, one solution is to dump all the files in the file cabinet and import some information onto the record about which ones should be attached and then run a script to link them. We were using box at the time so we also tried creating a shadow structure with the files in the right spot, which theoretically could work. I think in the end though, they just decided to clean up the file naming conventions, dump all the files somewhere, and put a little note on all the transactions that for documents older than XXXX date, you could find them here.