Comment 78 for bug 25830

So the first time we encounter text/rtf we should just dump it out as plaintext
and not give the user a chance to set up a reasonable handler for that
type? That's not reasonable.

In fact, that whole section of the RFC is incredibly bogus in my mind,
since most text/* types are in fact not human-readable (this includes
text/html and its ilk; people who code HTML for a living by hand notwithstanding).