format_from_path_extension

# File actionpack/lib/action_dispatch/http/mime_negotiation.rb, line 166
      def format_from_path_extension
        path = get_header('action_dispatch.original_path') || get_header('PATH_INFO')
        if match = path && path.match(/\.(\w+)\z/)
          Mime[match.captures.first]
        end
      end