Hi,
You can use the following,
<script type="text/javascript">
function fileCheck(obj) {
var fileExtension = ['jpeg', 'jpg', 'png', 'gif', 'bmp'];
if ($.inArray($(obj).val().split('.').pop().toLowerCase(), fileExtension) == -1)
alert("Only '.jpeg','.jpg', '.png', '.gif', '.bmp' formats are allowed.");
}
</script>
<asp:FileUpload ID="FileUpload1" runat="server" onchange="fileCheck(this);"/>