Fix test failure on macOS
Fix test failure on macOS

File file.ext was being guessed to be of mime type
"application/vnd.novadigm.ext" on macOS (vs "application/octet-stream"
everywhere else).

Note that using blog/dummy.xml doesn't work either, since that
get's grokked as "text/xml" on Windows vs "application/xml" on Linux
and macOS.

lektor/lektor,lektor/lektor,lektor/lektor,lektor/lektor