    # 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