Preserve non-ASCII characters in attachment names when downloading attachments

Now that we allow non-ASCII characters in attachment names, we should also offer nice filenames when the user tries to save them to their local computer. This should be done according to ftp://ftp.rfc-editor.org/in-notes/rfc2231.txt

This works in all modern browsers, but not in IE6 and IE7 where % encodings will be present in the filename.