Specifically,

_formatparam(): Teach this about encoded `param' arguments, which are
a 3-tuple of items (charset, language, value).  language is ignored.

_unquotevalue(): Handle both 3-tuple RFC 2231 values and unencoded
values.

_get_params_preserve(): Decode the parameters before returning them.

get_params(), get_param(): Use _unquotevalue().

get_filename(), get_boundary(): Teach these about encoded (3-tuple)
parameters.