How can I validate my field "key" before file will be uploaded using multer?

How can I validate my field "key" before file will be uploaded?

But I don't find any information about this. I use express, multer.

var upload = multer({
    storage: storage,
    limits: limits,
    fileFilter: fileFilter
}).fields([{ name: 'avatar', maxCount: 1 }, { name: 'key', maxCount: 1 }]);

router.post('/', (req, res) => {

    upload(req, res, function (err) {
        if (err) {
            console.log(err);
            // An error occurred when uploading
            res.send({result: err.message});
        }

        res.send({result: "OK"});
    });
});

via Dan