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Unfortunately, Excel can't cope with some of the more modern authentication systems being deployed on the web, so it's no longer as easy as providing a URL that spits out an HTML table, if you have to protect sensitive data behind a login (I think it can handle HTTP-Basic, but not anything requiring redirects; this may have changed in recent versions).

It's "application/vnd.ms-excel" for recent versions.

Yep. Believe you also need to set something like this: Content-Disposition: attachment; filename="foo.xls"