docs: private void attachJsonBody(final HttpServerExchange exchange, String string) throws IOException {
          Object body;
          if (string != null) {
              string = string.trim();
              if (string.startsWith("{")) {
                  body = Config.getInstance().getMapper().readValue(string, new TypeReference<Map<String, Object>>() {
                  });
              } else if (string.startsWith("[")) {
                  body = Config.getInstance().getMapper().readValue(string, new TypeReference<List<Object>>() {
                  });
              } else {
                  // error here. The content type in head doesn't match the body.
                  setExchangeStatus(exchange, CONTENT_TYPE_MISMATCH, "application/json");
                  return;
              }
              exchange.putAttachment(REQUEST_BODY, body);
          }
      }