Unicode attachment file name doesn't work on IE

[Cat]

Response.Append Header("Content-Disposition", "attachment ; filename=ѧ
У.txt");

This works correctly on Opera, FireFox or Safari. But on IE, the file
name gets broken. Setting HeaderEncoding to UTF-8 didn't solve it. If
I use Server.UrlEncod e, the file name is shown fine on IE but broken
like %XX%XX%XX... on other browsers.

What can I do?