Unable to validate image using Nodejs Api

I am working with Nodejs and using expressjs framework,Right now i am trying to upload image using Api but
i want two things

1) if image extension is not "jpg,png,gif" then error message should display

Here is my current code

var storage =   multer.diskStorage({
  destination: function (req, file, callback) {
    callback(null, './uploads');
  },
 filename: function (req, file, callback) {
     var sizeOf = require('image-size');
        var sizeOf = require('image-size');
        const originalname = file.originalname;
        const fileExt = originalname.split(".").pop();
       const excatName = originalname.split(".").slice(0, -1).join(".");
       console.log("mimetype is " + file.mimetype); // GETTING FILE EXTENSION NAME
       callback(null, excatName + "-" + Date.now().toString() + "." + fileExt);
      },
});
var upload = multer({ storage: storage }).single('userPhoto');
 const uploadavatar = function (req, res) {
    upload(req,res,function(err) {
        if(err) {
            return res.end("Error uploading file." + err);
        }
        res.end("File is uploaded");
    });
};