Having lobbied in favor of an aggregation construct, such as
multipart/attachment, I nonetheless believe that a generic,
additional "handling" header, such as Content-Disposition is a good
idea.
Content-type then would pertain ONLY to the descriptive information
of what is INSIDE the body part, whereas Content-Disposition --and
for some reason, I'd rather echo the term "handling"-- pertains
to the email-related processing of the part by the recipient agent.
I guess I agree with Nathaniel that this then becomes a better
place to put filename information.
Dave