# File lib/sinatra/respond_to.rb, line 164
      def format(val=nil)
        unless val.nil?
          mime_type = ::Sinatra::Base.mime_type(val)
          fail "Unknown media type #{val}\nTry registering the extension with a mime type" if mime_type.nil?

          @_format = val.to_sym
          response['Content-Type'] ? response['Content-Type'].sub!(/^[^;]+/, mime_type) : content_type(@_format)
        end

        @_format
      end