Grails没有为请求类型找到合适的HttpMessageConverter

Question

我正在尝试将文件上传到我的grails控制器，然后使用上传的文件调用外部服务。这是我的控制器代码：

  def detect() {
        def f = request.getFile('img')
        if (f.empty) {
            respond 'Image cannot be empty'     
            return
        }
        def rest=new RestBuilder()
        def resp = rest.post(...)
        respond resp.json
    }

但我得到以下例外：

| Error 2015-02-26 17:14:27,102 [http-bio-8080-exec-8] ERROR errors.GrailsExceptionResolver  - HttpMessageNotWritableException occurred when processing request: [POST] /Twins/detection/detect
Could not write request: no suitable HttpMessageConverter found for request type [org.springframework.web.multipart.commons.CommonsMultipartFile]. Stacktrace follows:
Message: Could not write request: no suitable HttpMessageConverter found for request type [org.springframework.web.multipart.commons.CommonsMultipartFile]

这段代码有什么问题？