Comment 3 for bug 383693

The problem seems to be, that this and "a few" other http servers response with the very generic Content-Type header value application/octet-stream, which could be any binary data and is therefore not allowed to open directly. Off course, the more appropriate MIME type application/pdf could be detected and sent by the server (file name extension, mime detection) but maybe Firefox shouldn't rely on the Content-Type data too much.
Indeed it doesn't, because it shows the correct file type "PDF Document" in the dialog. Where does this come from? Now, as soon as this dialog appears the file is downloaded to a temporary location (/tmp). I guess, mime-type detection is done on that file. Normally, the file header is enough for that.
So it would be nice to have the "open with" part of the dialog re-enabled as soon as the file type is detected on the client side, because surely this is not only a problem with PDF files.

I'm just a user, those are just some thoughts and they could be wrong.