[plt-scheme] creating MIME-encoded emails with PLT Scheme

I'm looking for a way to construct emails that are MIME-encoded using
PLT Scheme.  In particular, I'd like to send an email that contains
embedded images.  Using net/smtp and related libraries, getting a
plain email to send was a snap, but I don't see any libraries dealing
with encoding MIME.  (I've found what seems to satisfy the reverse
problem: http://download.plt-scheme.org/doc/html/net/mime.html#(mod-path._net/mime-unit).)

Thanks for any insight,
Rob