top-level MIME type is appropriate.
I suggest writing up a strawman definition for the new type and
publishing it as an Internet-Draft. It should probably specify:
a) the new top-level type name
b) names and definitions of any content-type parameters that apply
to all subtypes (like "charset" for text/*)
c) a description of what kinds of objects fit within the new
type and which ones do not
d) default user agent handling of unknown subtypes (if any)
...and any thing else that seems relevant.
It should also provide justification for the new type, which should
include one or more of:
a) a description of the hardware and/or transmission protocol requirements
which are needed to effectively present 3D geometry objects, beyond those
for normal MIME mail. (do you need a bitmapped display and the ability to
interact with the viewer? dialbox? 3d glasses?)
b) if a generic presentation module for all subtypes of the new type
were to be written, what features would it have that were common
to all subtypes?
Keith Moore