According to RFC2231 the named disposition (content disposition field) is provided by the MIME mechanism. The encoded parameter like the following:

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

is not behaving the way it should be.
The patch by rcoyner seems fine to me as it rectifies the wrong behaviour of _parseparam i.e. the counting issue of nested 'double quotes' and clears the unit tests.