Firefox, MSIE (starting with version 9), Opera, Konq and Chrome support; MSIE8 and Safari not support; others support is unknown – the encoding defined in RFC 5987.
Note that in
Content-Disposition: attachment; filename*=UTF-8''weird%20%23%20%80%20%3D%20%7B%20%7D%20%3B%20filename.txt
you got the encoding for the Euro character wrong; it’s unicode code point is not %80, fixing this should make it work everywhere except Safari (the correct encoding being %e2%82%ac).
Test case at:
http://greenbytes.de/tech/tc2231/#attwithfn2231utf8