Well, there's already application/vnd.ms-tnef and
application/mac-binhex40; of course that hides the media
type of what has been compressed.
Application.mac-binhex40 barely squeaked by because it is more than a
encoding/checksum/compression mechanism; it includes facilities for
representing the resource fork of Macintosh files in addition to the data fork.
The same applies to application/applesingle. But the preferred format for Mac
appication data is multipart/appledouble.
As for application/vnd.ms-tnef, while it is true it can be operated in a mode
where it incorporates both the content of the message as well as markup of that
content, it can also be operated in a mode where the content is external. This
sort of puts it in the applesingle/appledouble camp. Regardless, these issues
are dwarfed by the proprietary nature of this format.
And all this borders on the more general issue of how to represent complex
package formats in MIME, especially those that store information about their
content that MIME cannot. These are always judgement calls.
Ned