To follow up on what Roy T. Fielding said ...
Following the rules of RFC 1808, the base URL for resolving relative
URLs within that message (including the content) can be defined
by a Base: header field in the message or message part. This does
apply in this situation, does apply to MIME, and seems to be ignored
in this discussion.
I believe that there is valid reason for the message metadata to
provide information about either or both of two URLs for any
object conveyed by the message: an URL at the source and a
"disposition" URL at the destination. One "Base:" parameter
can't tell you both. I would [pending correction] read a "Base:"
header on an entity as connoting Source-Base. That is why I
suggested that "base" be used as a parameter under the
disposition header (Content-disposition:).
Al Gilman
PS: Factoring the disposition URL into a relative uri= parameter
and an absolute base= parameter lets us show how much of a local
location-pattern needs to be reproduced to preserve the links
among the parts of the patch of web so transplanted.