docs: public String generateFragment() {
          if (!DownloadSettings.usePostBack()) {
              return "";
          }
      	if (neverUpdate) return "";
          if (doesNotSupportPostMessage())  return "";
  
          StringBuilder buf = new StringBuilder();
          if(Jenkins.getInstance().hasPermission(Jenkins.READ)) {
              long now = System.currentTimeMillis();
              for (Downloadable d : Downloadable.all()) {
                  if(d.getDue()<now && d.lastAttempt+TimeUnit.SECONDS.toMillis(10)<now) {
                      buf.append("<script>")
                         .append("Behaviour.addLoadEvent(function() {")
                         .append("  downloadService.download(")
                         .append(QuotedStringTokenizer.quote(d.getId()))
                         .append(',')
                         .append(QuotedStringTokenizer.quote(mapHttps(d.getUrl())))
                         .append(',')
                         .append("{version:"+QuotedStringTokenizer.quote(Jenkins.VERSION)+'}')
                         .append(',')
                         .append(QuotedStringTokenizer.quote(Stapler.getCurrentRequest().getContextPath()+'/'+getUrl()+"/byId/"+d.getId()+"/postBack"))
                         .append(',')
                         .append("null);")
                         .append("});")
                         .append("</script>");
                      d.lastAttempt = now;
                  }
              }
          }
          return buf.toString();
      }