code: private void handleFile(final InputGroup group) {
      // Register a group start handler. The start handler will be called
      // with the file name. Then we can open the file.
      group.startHandler(new Handler<String>() {
        @Override
        public void handle(String fileName) {
          final File file = new File(tempDir, String.format("temp-%s-%s", UUID.randomUUID().toString(), fileName));
          input.vertx().fileSystem().open(file.getAbsolutePath(), new Handler<AsyncResult<AsyncFile>>() {
            @Override
            public void handle(AsyncResult<AsyncFile> result) {
              if (result.succeeded()) {
                handleFile(file.getAbsolutePath(), result.result(), group);
              } else if (exceptionHandler != null) {
                exceptionHandler.handle(result.cause());
              }
            }
          });
        }
      });
    }
docs: Handles a group input file.