Apply Below given Custom Validator on file Upload Control
<asp:CustomValidator CssClass="asterisk" ID="CustomValidator1" runat="server" Display="Dynamic" ControlToValidate="FileUploadCtrl" ErrorMessage="Please browse file to Upload" SetFocusOnError="true" ValidateEmptyText="True" ValidationGroup="ValGrp"
ClientValidationFunction="CheckFile">
</asp:CustomValidator>
Write below javascript
function CheckFile(source, arguments) {
var path = document.getElementById('<%= FileUploadCtrl.ClientID %>').value;
if (path != "")
arguments.IsValid = true;
else
arguments.IsValid = false;
}
It will definitely help you