Sorry I couldn't make to to IETF in Columbus...
From: Greg Vaudreuil <gvaudre(_at_)cnri(_dot_)reston(_dot_)va(_dot_)us>
Minutes of the RFC 822 Message Extensions Working Group
Representation of Filenames in Message/External-body.
The inclusion of filenames in the content-type headers has the effect
of requiring that all filenames be 7 bit ASCII. The Working Group
discussed the likelihood that new operating systems will require a
richer character set for filenames and possiblity that when this
occurs the current filename mechanism may not be adequate. ...
"Likelihood"? "Possibility"? "When this occurs"?
The Macintosh OS uses an 8 bit charactrer set for file names and
disallows only the character colon (":"). I can even have embedded
nulls or other weirdness in file names on my Macintosh.
NT uses full 16 bit UNICODE for filenames, in case no one has noticed.
I am sure there are MANY other examples.