I've fixed the `rfc2231-parse-string' function in both the trunk
and the v5-10 branch so that they might be decoded.

That the file name "a*.txt" doesn't appear in the former MIME
button is due to a bug of Gnus, because RFC2183 and RFC2045 does
not request that `*'s should be encoded in parameter values.
ARISAWA-san quoted the related specifications as follows:

RFC2183:
========
     filename-parm := "filename" "=" value
[...]
   NOTE ON PARAMETER VALUE LENGHTS: A short (length <= 78 characters)
   parameter value containing only non-`tspecials' characters SHOULD be
   represented as a single `token'.  A short parameter value containing
   only ASCII characters, but including `tspecials' characters, SHOULD
   be represented as `quoted-string'.

RFC2045:
========
     value := token / quoted-string

     token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
                 or tspecials>

     tspecials :=  "(" / ")" / "<" / ">" / "@" /
                   "," / ";" / ":" / "\" / <">
                   "/" / "[" / "]" / "?" / "="

As for the latter case, that the Japanese file name isn't decoded
is due to a bug of a certain mailer (he wrote that some version
of Thunderbird does it), since (also he wrote) RFC2231 specifies
that `*'s should be encoded in the `extended-parameter'.  Even
so, because there is no reason that they should not be decoded,
Gnus now decodes such parameter values, though.

Regards,