# File lib/mail/message.rb, line 1283
    def add_charset
      if body.only_us_ascii?
        header[:content_type].parameters['charset'] = 'US-ASCII'
      else
        warning = "Non US-ASCII detected and no charset defined.\nDefaulting to UTF-8, set your own if this is incorrect.\n"
        STDERR.puts(warning)
        header[:content_type].parameters['charset'] = 'UTF-8'
      end
    end