Comment 9 for bug 589236

Given that on Windows FF3 offers immediately to open such a download with the right application (testdownload above is suggested to be opened with Adobe Reader) it's seems to be ok to fall back to the extension mapping and ignoring the application/octet-stream mimetype returned by the webserver.

So the solution to this issue on Linux would be to make sure that (in addition to mime-types/mailcap also Gnome VFS should return a default application when there is none for the server defined mime-type.