File type validation with Javascript

Question

I have a question regarding to JavaScript validation. I am validaing the whenever my scripts runs, it validates but also the action

Answer 1

var _URL = window.URL || window.webkitURL;
    $("input[type=file]").change(function(e) {
        var file;
            if ((file = this.files[0])) {
                var img = new Image();
                img.onload = function () {
                    // do to on load
                };
                img.onerror = function () {
                    alert("valid format " + file.type);
                };
                img.src = _URL.createObjectURL(file);
            }
    });