When you click on a link and get a file with an unknown mime type, (or for a file with an associated app once that happens), have an option for "Treat file as if it were a file of type X". For example I could view files as text or html right in the browser if the server doesn't know a mime type for the file, or sends a mime type for an app I don't have. Also a persistent "View all files of type X as if they were type Y" where Y can be HTML, or any other mime type that is displayed right in the browser. Or one that would launch an external app.

this is probably covered by bug 11521, bug 18219, bug 38374 or bug 43583 *** This bug has been marked as a duplicate of 11521 ***

ver dupe