http://www.ietf.org/rfc/rfc2231.txt
This memo defines extensions to the RFC 2045 media type and RFC 2183
disposition parameter value mechanisms to provide
(1) a means to specify parameter values in character sets
other than US-ASCII,
(2) to specify the language to be used should the value be
displayed, and
(3) a continuation mechanism for long parameter values to
avoid problems with header line wrapping.
This memo also defines an extension to the encoded words defined in
RFC 2047 to allow the specification of the language to be used for
display as well as the character set.