code: public Http4K publishFileSystem( String hostName, String urlPath, int port, File root ) {
          if ( ! root.exists() )
              root.mkdirs();
          if ( ! root.isDirectory() ) {
              throw new RuntimeException("root must be an existing direcory:"+root.getAbsolutePath());
          }
          Pair<PathHandler, Undertow> server = getServer(port, hostName);
          server.car().addPrefixPath(urlPath, new ResourceHandler(new FileResourceManager(root,100)));
          return this;
      }
docs: publishes given file root
  @param hostName
  @param urlPath - prefixPath (e.g. /myapp/resource)
  @param port
  @param root - directory to be published