Jesse Glick ah ok ... I didn't see that. I did look, but I was filtering on "Bug" only (not "Feature" since it looked as though it was already supposed to be a feature) and so didn't see it when I scanned the list.
There was code in there for handling File Parameters, though I have my doubts that it ever worked since the multipart formdata handling was missing (no tests for it either). So were you thinking of doing something similar to the BuildWrapper stuff used in Freestyle builds (would require core changes I suspect)?
In any case, maybe we could have short and longer-term fixes perhaps. What I put in this WiP PR is horribly hacky, but gets it working. It could be cleaner if Stapler allowed better access to parsed formdata. Longer-term we could fix it better with whatever you have in mind?