The same logic was duplicated in three places. Factor it
out so that we can add further fallbacks in a single place.

423b1c69be7828c331a47467b66217d190e916b9, which in turn
causes all content to be served as text/plain. Let go decide
what the mime type is (e.g. text/html instead of text/plain).

423b1c69be7828c331a47467b66217d190e916b9, which in turn
causes all content to be served as text/plain. Let go decide
what the mime type is (e.g. text/html instead of text/plain).