jq 小笔记，上传判断其格式

<script type="text/javascript">
        jQuery(function () {


            var SpanShow = function () {
                var refNo = jQuery("#txtOrderRefNo").val();
                var uploadfile = jQuery("#FileAwSizeUpload").val();

                if (refNo == "" || refNo == null) {
                    jQuery("#s_order").show();
                    return false;
                }
                if (uploadfile == "" || uploadfile == null) {
                    jQuery("#s_FileUpload").show();
                    return false;
                }

                var extStart = uploadfile.lastIndexOf(".");
                var ext = uploadfile.substring(extStart, uploadfile.length);
                if (ext != ".pdf") {
                    alert("只能上传pdf格式");
                    return false;
                } 
            }

            jQuery("#btnUpload").click(SpanShow);
        });
    </script>