MIME currently doesn't (and shouldn't) try to solve the problem of how
to present each of the body parts of a message. The reason: we don't
yet know how to do this well, and more experimentation is needed. I
would recommend that those wanting to work on solutions define their
own experimental MIME content-types, which reference the content-ids
of other MIME body parts, and define how those body parts are to be
presented. (NeXTmail essentially does this now -- the first part of a
message is RTF that contains text mixed with references to sounds,
graphics, etc., that appear in other body parts.)
Of the "quick" fixes to MIME that allow "attachments" to be
distinguished from "inline" body parts, I prefer simply adding an
(optional) parameter to Content-type. This seems like a less
drastic change to MIME, and one which we can more easily deprecate (or
simply forget) later when we have a better way of defining the
presentation of the body parts of a message.
If it turns out that we want presentation information in a different
header, then I would recommend putting the "filename" parameter in the
content-presentation header also.
Keith