Actually, I think Jay Weber is closest to the truth. We added
`multipart/mixed' to handle problems of compound document markup,
without really being conscious of it, and then didn't think hard enough
about the problem. Multipart/mixed is missing a specification of just
*how* the different parts go together (that is, is this picture embedded
in that text, or what?), because that would be straying into the realm
of document formats. Multipart/parallel (meaning simply ``here're a
bunch of parts'') and multipart/alternative seem to have reasonable
semantics, though.
Bill