<script type="text/javascript" language="javascript">
function validate() {
var fileExt= document.getElementById('<%=FileUpload1.ClientID%>').value;

var reg = /^(([a-zA-Z]:)|(\\{2}\w+)\$?)(\\(\w[\w].*))+(.xls|.xlsx|.XLS|.XLSX)$/;
if (uploadcontrol.length > 0)
{

if (reg.test(fileExt))
{
 return true;
}
else
{
 alert("Only .Xls, .xlsx files are allowed!");
 return false;
}
}
} 

<script language="javascript" type="text/javascript">
    function filterFiles() {
        var path = document.getElementById('<%= FileUpload1.ClientID %>').value;

        if (path.length < 1) {
            alert("Please select a file");
            return false;
        }

        var extension = path.substring(path.lastIndexOf('.') + 1).toLowerCase();

        var allowedExt = new Array();
        allowedExt[0] = "jpg";
        allowedExt[1] = "jpeg";
        allowedExt[2] = "png";
        allowedExt[3] = "gif";
        allowedExt[4] = "doc";
        allowedExt[5] = "docx";
        allowedExt[6] = "ppt";
        allowedExt[7] = "pdf";
        allowedExt[7] = "txt";
        allowedExt[8] = "xls";
        allowedExt[9] = "xlsx";

        for (var i = 0; i < allowedExt.length; i++) {
            if (extension == allowedExt[i]) {
                return true;
            }
        }
        alert("File type: " + extension + " is not allowed!!!");
        return false;
    }
    </script>

<asp:FileUpload ID="FileUpload1" runat="server" /><br />
<asp:Button ID="btnUpload" runat="server" Text="Upload" 
   OnClientClick="return filterFiles()" onclick="btnUpload_Click" />