Fix 0010494: support UTF-8 attachment filenames

RFC2231 gives us the ability to define a filename in the
Content-Disposition header that is encoded in UTF-8. Unfortuantly older
browsers such as Internet Explorer 7 do not support RFC2231. Therefore
this implementation provides a fallback mechanism that works with most
major browsers.

See http://greenbytes.de/tech/tc2231/ for more information on what
techniques we have available, and why I've patched this using attfnboth2
fallback technique.