code: protected boolean checkMultipart(Invocation inv) throws MultipartException {
          if (inv.getRequest().getMethod() == null) {
              throw new NullPointerException("request.method");
          }
          if (this.multipartResolver.isMultipart(inv.getRequest())) {
              if (inv.getRequest() instanceof MultipartHttpServletRequest) {
                  logger.debug("Request is already a MultipartHttpServletRequest");
                  return true;
              } else {
                  if (!inv.getMethod().isAnnotationPresent(SuppressMultipartResolver.class)) {
                      inv.setRequest(this.multipartResolver.resolveMultipart(inv.getRequest()));
                      return true;
                  }
              }
          }
          return false;
      }
docs: ------------------------------------------------------