Value of Content-Disposition header:
Content-Disposition: attachment; filename*0*=ISO-8859-15''euro-sign%3d%a4; filename*=ISO-8859-1''currency-sign%3d%a4
'attachment', specifying a filename of
the simple RFC2231/5987 format, and
euro-sign=€ in RFC2231-with-continuations format.
A UA that supports could pick either, or ignore both because of the ambiguity.