Comment 81 for bug 25830

I think it's a fair point that defaulting to show-as-plain-text would not give
the user the opportunity to set up a custom handler for text/rtf or whatever.
On the other hand the current behaviour does not give the user the opportunity
to view a document in the browser. So you could weigh up which of the two is worse.

But perhaps you don't want to do anything on this until it can be done _right_
(which I assume means an extra UI preference and a 'view in browser' option on
the helper app chooser). That sounds entirely reasonable.