> Would there be any practical implications one way or the other?

I think it's a matter of documenting the behavior, and the caller should do previous processing beforehand to consider opaque URIs.

> What happens (and what do we want to happen) if there's a data URI in a fileset that we want to store?

Yeah, I considered the issue. I think a reasonable approach w/b to leave data URIs out of the filesets. If a user wants to serialize inlined `data:` content to files, we should offer some utilities which would return a fileset doc representing the new files, but this shouldn't occur transparently in `px:fileset-store` IMO.

Currently, `px:fileset-store` fails if a `d:file` has a `data:` URI, which can be considered as a low-priority bug.