Javascript image upload function

script.js

function ajax_file_upload(file_obj) {
    if(file_obj != undefined) {
		$("h2").text("Starting upload...");
        var form_data = new FormData();
        for(i=0; i<file_obj.length; i++) {
			if (!isImage(file_obj[i].name)) {
				return failValidation("ext");
			}
			if (!validateSize(file_obj[i].size)) {
				return failValidation("size");
			}
            form_data.append('file[]', file_obj[i]);  
        }
        $.ajax({
            type: 'POST',
            url: 'upload.php',
            contentType: false,
            processData: false,
            data: form_data,
			dataType: 'json',
			xhr: function() {
                var xhr = new window.XMLHttpRequest();
                xhr.upload.addEventListener("progress", function(evt) {
                    if (evt.lengthComputable) {
                        var percentComplete = evt.loaded / evt.total;
                        //console.log(percentComplete);
                        $('h2').text('Uploading... ' + (Math.round(percentComplete * 100)) + '%');
                    }
                }, false);
                return xhr;
            },
			success: function (response) {
				for(var index = 0; index < response.length; index++) {
					$("h2").text("SUCCESS: "+response.length+" file(s) uploaded!");
					var src = response[index];
					$('#preview').append('<a href="'+src+'" target="_blank"><img src="'+src+'" width="100px;" height="100px"></a>');
				}
			}
        });
    }
}