to
# The value MAY include any number of parameters, depending
# on the specification of the Media Type in the IANA Registry [IANA-MT].
Many media type registrations have both required and optional
parameters. A MIME type which requires a parameter is meaningless
without the parameter, and there are many other parameters other
than 'charset'.
It is also true that most 'text' types allow a 'charset' parameter,
and you can point this out if you want; the current wording, though,
seems to imply that parameters other than 'charset' are not allowed,
which would be wrong.
Larry
-- http://www.parc.xerox.com/masinter