小程序使用wx.chooseImage上传本地相册图片到后端，后端用的MultipartFile接收，报错：the request was rejected because no multipart boundary was found

org.springframework.web.multipart.MultipartException: Failed to parse multipart servlet request;
 nested exception is java.io.IOException: org.apache.tomcat.util.http.fileupload.FileUploadException: 
    the request was rejected because no multipart boundary was found