Comment 3 for bug 39136

All the common text/* types that we don't already render as text/plain
are not something the user wants to see by default (text/rtf is the only one
I can think of).

And no, a helper app does not currently override internal handling.
Furthermore, the architecture to do it is not in place and putting
it in place would also allow the "view in browser" option.

Note that once a user selects to open a type in the browser, the decision
would get remembered, so the only difference is that the user gets explicit
UI once to decide on a course of action.