# File lib/action_mailer/vendor/tmail-1.2.3/tmail/interface.rb, line 792
    def set_content_type( str, sub = nil, param = nil )
      if sub
        main, sub = str, sub
      else
        main, sub = str.split(%r</>, 2)
        raise ArgumentError, "sub type missing: #{str.inspect}" unless sub
      end
      if h = @header['content-type']
        h.main_type = main
        h.sub_type  = sub
        h.params.clear
      else
        store 'Content-Type', "#{main}/#{sub}"
      end
      @header['content-type'].params.replace param if param
      str
    end