node处理post请求上传图片，为什么会写两个图片，readfile的时候已经创建了一个图片文件

var upload = multer({
    dest: 'upload/'
});

router.post('/', upload.any(), function (req, res, next) {
    console.log('上传的文件')
    console.log(req.files[0]); // 上传的文件信息
    var extName = ''; //后缀名
    switch(req.files[0].mimetype) {
        case 'image/pjpeg':
        extName = '.jpg';
        break;
      case 'image/jpeg':
        extName = '.jpg';
        break;
      case 'image/png':
        extName = '.png';
        break;
      case 'image/x-png':
        extName = '.png';
        break;
        default :
        extName = ''
    }
    var des_file = "/" + req.files[0].originalname;
    fs.readFile(req.files[0].path, function (err, data) {
        console.log(data)
        fs.writeFile(req.files[0].path+extName, data, function (err) {
            if (err) {
                console.log("失败")
                console.log(err);
            } else {
                response = {
                    message: 'File uploaded successfully',
                    filename: req.files[0].originalname
                };
                console.log(response);
                res.end(JSON.stringify(response));
            }
        });
    });
});