code: public Http4K publishFileSystem( String hostName, String urlPath, int port, FileResourceManager man ) {
          if ( ! man.getBase().isDirectory() ) {
              throw new RuntimeException("root must be an existing direcory:"+man.getBase().getAbsolutePath());
          }
          Pair<PathHandler, Undertow> server = getServer(port, hostName);
          server.car().addPrefixPath(urlPath, new ResourceHandler(man ));
          return this;
      }
docs: FIXME: exposes Undertow class