Guido is completely right. For an archive, you could indeed do
everything relative to a root. For that, you might define
multipart/archive, with the understanding that all occurences of
"filename= xxx" in the parts is to be consider relative to, say, the
"filename=xxx" in the multipart content-type. That's a very simple and
elegant model, and it is true to the spirit of multipart subtypes in
that if people choose to treat it as multipart/mixed it will still work
reasonably. In fact, that's simple and elegant enough that I'm tempted
to add it to the draft. But I've been fighting such temptations
recently....