解决方法：后端下载controller层添加

  String str = new StringBuilder();
   str.append("attachment; filename=")
         .append(filename) 
        .append(";")
       .append("filename*=")
         .append("utf-8''")
        .append(filename); //filename不是真实文件名，需要做百分号编码处理
//添加响应头 
response.addHeader("Access-Control-Expose-Headers", "Content-Disposition,download-filename");     
//添加响应头内容
response.setHeader("Content-disposition", str.toString());
response.setHeader("download-filename", filename);

uni.downloadFile记得设置请求头