To Upload only specific extension file using javascript

JAVASCRIPT

<script type="text/javascript">
        function GetFileName(val) {
            jQuery('#<%=TextBox1.ClientID %>').val(val).trigger('change');
            return true;
        }
    </script>



    <script type="text/javascript">

        function ValidateFile(Source, args) {
            var fuData = document.getElementById('<%= importdata.ClientID %>');
            var FileUploadPath = fuData.value;
            var Extension = FileUploadPath.substring(FileUploadPath.lastIndexOf('.') + 1).toLowerCase();
            if (Extension == "txt") {
                args.IsValid = true;
            }
            else {
                args.IsValid = false;
                document.getElementById('<%= importdata.ClientID %>').value = "";
            }
        }




ASPX


<asp:FileUpload ID="importdata" runat="server" />
 <asp:CustomValidator ID="CusFileType" runat="server" ClientValidationFunction="ValidateFile"
ControlToValidate="importdata" Display="Static" ErrorMessage="Please Select a Text File" ForeColor="Red" SetFocusOnError="True" />