> Is there a way to get signatures or encryption working on messages
> that contain attachments, or does mml2015.el just not do it yet?

It should work, does it not?  Let's see, I'll attach a dummy file and
sign this mail..  *previewing it* No, this doesn't look right, only
the text was signed.  Well, you can add "sign=pgpmime" to the
attachment MML tag as well, but then you'll get two multipart/signed
structures, which probably no other mail reader than Gnus would
handle.

Ah, you can do:

<$mml sign=pgpmime>
text...
<$part filename="~/foo">

and then you only get one top-level multipart/signed part containing a
multipart/mixed part with the text and the file.  But somehow the
attribution ("Josh Huber <huber@alum.wpi.edu> writes:") ends up inside
MIME headers.  Perhaps this breaks the signature as well.  Let's see..