I don't think it is (currently).

I seem to remember Evan mentioning the (maybe theorical/future) possibility of actually specifying attachments, but I guess there's nothing tangible.

The current way we do this isn't really "attaching"; it's creating a "image object", or an "audio object" etc, which just happens to have an optional "description" field, making it look like a "note with image attached". I know, semantics xD