Multipart/mixed and HTTP

Hi,

I noticed that the Mozilla derived browser support the
multipart/mixed mime type, Opera with a few bugs and others,
such as Internet Explorer and Safari (on MacOS X) don't.

Is multipart/mixed something that is meant to be an accepted
MIME type for web browsers, or is this something extra implemented
by Mozilla? I am not sure where to find the right document, if
there is one, where this is covered.

Thanks

André-John