Comment 80 for bug 25830

My suggestion would be to have a back-end pref with a list of types that are
handled as text/plain. (My pet peeve is trying to view dtd files at w3c.org)
This would save the extra pref in the ui, but still give advanced users an
option to change it through about:config or the .pref file. This pref should
come preloaded with things like text/css, text/tab-separated-values,
application/xml-dtd, and others that are known human-readable file types.