This isn't exactly what you want, but it works.

When you open a Word document that's an email attachment by double-clicking
it, make changes, then save/close the document, those changes are saved in
the email's copy of the attachment. If you then click Reply, it won't
include the attachment. However, if you click Forward, it does include the
changed attachment. You can change FW to RE if you  want to avoid confusing
your correspondent (unless you're forwarding it to a 3rd party, in which
case FW is fine).

I often use this technique. I also save the attachment to a non-temporary
location, so that I'll have it where I'd ordinarily expect to find it; I
also have a copy of the changed file as a attachment to that message in my
Sent items, as well.