It's badly documented, but it appears to be there for me, but in the "UseOuterEnvelopeLabel" vi (see below). This is because that is where I envision it being useful, with a requestor attaching something to the reply.
But you may be using the "Add Attachment" vi, directly. This is still on disk, just not in the palettes. I will add it back into the palettes (issue 5).