> who knows where it is used?

As far as I can tell, it is only ever used by =(eww-submit ...)= from
=lisp/net/eww.el=.  That's the only place it's used in all of emacs,
and every reference I can find on the web.

I've tested the heck out of it now, and also used firefox, chromium,
and curl to generate http messages for comparison.

Those tests of the different versions of
=mm-url-encode-multipart-form-data=, as well as the http messages
generated by firefox, chromium, and curl can be seen here

https://git.sr.ht/~ozzloy/emacs-bug-63941/tree/53a7949a5db21c456c1da3b4add29343c3d02137/item/mm-url-tests.el

The patch I have attached to this email generates output that matches
firefox, chromium, and curl.  It also includes a bunch of tests for
the included version of =mm-url-encode-multipart-form-data=

If there's some change that would make the patch a better fit, let me
know.  I'm happy to modify it.