Ref T5187. This definitely feels a bit flimsy and I'm going to hold it until I cut the release since it changes a couple of things about Workflow in general, but it seems to work OK and most of it is fine.
The intent is described in T5187#176236.
In practice, most of that works like I describe, then the phui-file-upload behavior gets some weird glue to figure out if the input is part of the form. Not the most elegant system, but I think it'll hold until we come up with many reasons to write a lot more Javascript.
