fileupload control

Question

hi here i'm using this script for validate the image file like gif,jpg,

<script type="text/javascript">

        $(function() {

            $('#<%=PhotoUpload.ClientID %>').change(function() {

                var fileExtension = ['jpeg', 'jpg', 'png', 'gif', 'bmp'];

                if ($.inArray($(this).val().split('.').pop().toLowerCase(), fileExtension) == -1) {

                    alert("Only '.jpeg','.jpg', '.png', '.gif', '.bmp' formats are allowed.");

                }

            })

        })

    </script>

but after displaying alert message when i click ok fileupload control takes that file please help me