The message method get_params() and the related get_param() and
get_filename() do not properly decode an RFC 2231 encoded parameter such
as the following:

Content-Disposition: inline;
 filename*0="Re: [Mailman-Users] Messages shunted with \"TypeError: ";
 filename*1="decodingUnicode is not supported\".eml"

This is because the message helper function _parseparams() mistakenly
thinks the second semicolon is inside a quoted string because it counts
the quoted (escaped) quote and sees an odd number.

The attached patch will fix this.