how to add validation of upload file using asp c#
I want to validate .xls|.xlsx file format
<asp:FileUpload ID="FileUpload" runat="server" />
<br /><asp:RequiredFieldValidator ID="RequiredFieldValidator1" ErrorMessage="Please choose a file." style="color: red; visibility: visible;" ControlToValidate="FileUpload"
runat="server" Display="Dynamic" ForeColor="red" Font-Size="20px" />
<asp:Button Text="Upload" OnClick = "Upload" runat="server" ID="Button1" Height="20px" style="text-align: center" Width="116px" CausesValidation="false" />
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" ValidationExpression="([a-zA-Z0-9\s_\\.\-:])+(.xls|.xlsx)$"
ControlToValidate="FileUpload" runat="server" ForeColor="red" Font-Size="20px" ErrorMessage="Please select Either .xls or .xlsx File with correct format." Display="Dynamic" />
