Safari displays the Firefox 1.5 distribution DMG as garbage text instead of downloading it, despite the correct mime-type. We have several packet traces and other debugging info at https://bugzilla.mozilla.org/show_bug.cgi?id=318199
-[WebLoader connection:didReceiveResponse:] receives 'text/html' from NSURLConnection - so it is the latter's content sniffer that breaks things. Should likely be closed as INVALID (bug in Apple system software), and handled via <http:// bugreporter.apple.com>, but I'll leave this for someone else to verify. The problem itself is of course very real.
That's right. This should be reported at http://bugreport.apple.com because it's a bug in lower-level system software that WebKit relies on. I believe there's already a bug report about this general issue in Radar and we're doing what we can to get it fixed as soon as possible. But please file another if you would like.