It might be worthwhile noting that the original example that led to
this discussion, where an attachment was shown in the middle of a
paragraph, is not actually representable in MIME with any variant of a
Content-Disposition header.
In the original MIME document, it was assumed that that this
functionality was provided, and early versions of the Andrew driver for
MIME provided this capability by what was essentially a hack: if the
previous text object didn't end with a newline, then the following
object was assumed to be embedded inline; if it did end with a newline
than it effectively started its own paragraph. The problem is that
there are a lot of perfectly acceptable, but different, ways of reading
the MIME spec. This particular one had a lot of implied semantics that
matched Andrew's document model but was never stated explicitly in the
MIME spec.
It turns out that encoding this kind of presentation information
involves quite a bit more work in defining what your compound document
model is. The MIME authors wisely avoided diving into this can of
worms (I bet Nathaniel's ODA experience had something to do with that).
Terry