As per RFC 2184 & 2231, if one wants the web browser to properly display a filename that contains Unicode characters in the File Download dialog box, you must encode the filename as URL-encoded UTF-8 in the Content-Disposition field.
Let's say I want to download an image named 2008年8月8日.jpg
The server should send down the following header
Content-Disposition: attachment; filename*="2008%e5%b9%b48%e6%9c%888%e6%97%a5.jpg"
In Firefox it works as expected (Note: it appears Firefox supports multiple ways of encoding international filenames. For example, the following header also gives the desired result.
Content-Disposition: attachment; filename="2008\345\271\2648\346\234\2108\346\227\245.jpg"
In IE, if you omit it the asterisk after the filename filed it works (like so)..
Content-Disposition: attachment; filename="2008%e5%b9%b48%e6%9c%888%e6%97%a5.jpg"
I've yet to see this work in Safari using any web application (I've tried Gmail, Hotmail, Windows Live Skydrive) or any of the above methods.
*** This bug has been marked as a duplicate of 15287 ***