We battled with this, too. In essence, IE ignores the MIME setting under some circumstances, especially when based on the file name it thinks they're wrong. The solution is complex.

Content-Disposition: attachment; filename="blah.xls"

works as a start. IE 5 and 5.5 also should get Content-Type: application/download.

And make sure the URL contains the filename after the last slash! This we found would trick IE into using the correct filename when saving.

Finally, try making your web server respond as HTTP/1.0 for IE 5. We found some comments a month or three ago that suggested IE forgets how MIME handling works when it receives HTTP/1.1, but we haven't tested this.

Wade.