Check Image extension using input type file in html using Jquery

Here is the code :

$fileinput1.on("change", function () {
                var input = this;
                var url = $(input).val();
                var ext = url.substring(url.lastIndexOf('.') + 1).toLowerCase();

                if (input.files && input.files[0] && (ext == "png" || ext == "jpeg" || ext == "jpg")) {

                    var FR = new FileReader();
                    FR.onload = function (e) {
                        var imagetocheck = new Image();
                        imagetocheck.src = e.target.result;
                        imagetocheck.onload = function () {
                            var width = this.width;
                            var height = this.height;
                            if (width == 50 && height == 50) {
                              
                            }
                            else {
                               
                                alert('Invalid Dimensions - Only image with Height-50px and Width 50px is accepted !!');
                            }
                        };
                    };
                    FR.readAsDataURL(this.files[0]);
                }
                else {                  
                    alert('Invalid extension');
                }
            });