Comment 5 for bug 15224

In my opinion, plug-in-registered file extensions (4) should have priority over
OS mappings (3). Actually, I think OS mappings should be ignored altogether when
selecting a plug-in.

If OS mappings are used, then the plug-in selected will sometimes depend on what
(not-necessarily-browser-related) applications the user happens to have
installed on his computer, and even on the order in which they were installed,
and how they are configured. Although this won't apply in most cases (because a
Content-Type header will be present), situations like this can be very
frustrating for plug-in developers and users.

An exception could perhaps be made for the "file:" URL scheme, but it may not be
worth the extra confusion.