the argument I was making was really:
If I get multipart/foo, and don't understand /foo, I have to display
it as multipart/mixed, and then I find application/foo, and have
to display *that* as application/octet-stream, which may or may not
strike the reader as "a glop of cybercrud".
If I get multipart/related;header=application/foo, I can display this as
multipart/mixed, with the proviso that when I encounter application/foo,
I can display it as
(header application/foo ignored)
(or equivalent) which may be a little less "cybercrud" than the default
handling of application/octet-stream.
(In Metamail, I think the default is to ask the recipient if he wants
to save it to a file, view it as ASCII or ignore it, not?)