uploading files from react to node js with multer

I want to upload files from the form data in react which is being posted by axios like this.

const addNewProduct = () => {
        const newProduct = {
            name: name,
            cost: cost,
            size: size,
            color: color,
            material: material,
            discount: discount,
            description: description,
            category: category
        };

        const nulls  = Object.values(newProduct).filter(p => p === null);

        if(nulls.length === 0 && images.imageFiles) {
            let productFormData = new FormData();
            productFormData.append('productInfo', JSON.stringify(newProduct));
            productFormData.append('productImages', images.imageFiles);
    
            const addUrl = "http://localhost:8080/cpnl/addproduct";
            axios({
                method: "POST",
                url: addUrl,
                data: productFormData,
                headers: { "Content-Type": "multipart/form-data" }
            })
                .then((response) => {
                    console.log(response.data.msg);
                })
                .catch((response) => {
                    console.error(response);
                });
        }else {
            Notiflix.Notify.Warning("Check your inputs!");
            console.log(nulls);
            console.log("product: \n" + JSON.stringify(newProduct));
        }
};

then I want to upload images with multer to images folder. this is my code:

const storage = multer.diskStorage({
destination: "./public/images",
filename: (req, file, cb) => {
    cb(null, file.fieldname + '-' + Date.now() + path.extname(file.originalname));
}
});

const upload = multer({
    storage: storage,
    limits:{fileSize: 1000000},
    fileFilter: function(req, file, cb){
        checkFileType(file, cb);
    }
}).array("productImages", 5);

function checkFileType(file, cb) {
  // Allowed ext
  const filetypes = /jpeg|jpg|png/;
  // Check ext
  const extname = filetypes.test(path.extname(file.originalname).toLowerCase());
  // Check mime
  const mimetype = filetypes.test(file.mimetype);

  if(mimetype && extname){
    return cb(null,true);
  } else {
    cb('Error: Images Only!');
  }
}

//receive form data from front-end and add new product to database
router.post('/addproduct', async (req, res) => {
    upload(req, res, (err) => {
        if(err) {
            res.status(400).json({
                msg: err
            });
        } else {
            if(req.files == undefined) {
                res.status(400).json({
                    msg: "Error: No file selected! please contact the developer."
                });
            } else {
                data = req.body.productInfo;
                res.status(200).json({
                    msg: "Files uploaded!"
                });
                console.log( "images: " + req.files);
                console.log("data" + data);
            }
        }
    });
});

first problem: I'm getting image files inside req.body.productImages and not inside req.files

second problem: when I send the request node js throws me this error:

TypeError: upload is not a function

why everything is messed up!?

Edit: I restarted the server and now I'm getting data but the files are not being uploaded. no error is shown.

UPDATE: second problem fixed