This is a little odd (it's more complex for one!), but means we can
reuse the internal API interface in other classes, which is useful for
the data provider refactor I'm about to do.
This is much nicer in Java 17 :D (records, ServiceLoader.stream()),
but such is the perils of still targetting 1.16.
We now fuzz UploadFileMessage, generating random files and checking they
round-trip correctly.
The joy of having a long-lasting refactor branch with an absolutely
massive diff, is that you end up spotting bugs, and then it's a massive
pain to merge the fix back into trunk!
