code: public static MultipartContent multipart(@DelegatesTo(MultipartContent.class) Closure closure) {
          MultipartContent content = new MultipartContent();
          closure.setDelegate(content);
          closure.call();
          return content;
      }
docs: Configures multipart request content using a Groovy closure (delegated to {@link MultipartContent}).
  
  @param closure the configuration closure
  @return a configured instance of {@link MultipartContent}