Handle mime types correctly for files where the extension is capitalized

Fix for https://github.com/playframework/playframework/issues/3650
Turkish i bug, if the platform locale is set to Turkish, GIF will become gıf (note no dot over the i), which will cause this to fail to detect that it should be image/gif.  Always explicitly chose a locale when converting cases, unless you want a locale specific conversion.  Eg `ext.toLowerCase(Locale.ENGLISH)`.