RegularExpressionValidator and ValidatorCalloutExtender problem after postback

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ASP.NET

Hi All

I have a File uploader control and a button called ADD. When I click the ADD buttion, the selected file format(PDF) will be validated using RegularExpressionValidator and ValidatorCalloutExtender. It is working fine. The problem is, the validation error message is being displayed after postback too. Could any one help on this issue?

I am using the below code

<asp:FileUpload ID="fuFileUpload" runat="server" />
                                <asp:Button ValidationGroup="fileUploadValidation" ID="btnUpload" OnClick="btnUpload_Click" runat="server"
                                    Text="Add" class="seaBtn" />
                                <asp:RegularExpressionValidator SetFocusOnError="true" runat="server" ID="revUpload" ControlToValidate="fuFileUpload"
                                    Display="None" ValidationGroup="fileUploadValidation" ErrorMessage="Only PDF or JPG files are Allowed."
                                    ValidationExpression="^(([a-zA-Z]:)|(\\{2}\w+)\$?)(\\(\w[\w].*))(.jpg|.JPG.|.pdf|.PDF)$" />
                                <asp:ValidatorCalloutExtender ID="vceUpload" runat="server" TargetControlID="revUpload"
                                    PopupPosition="BottomLeft" WarningIconImageUrl="~/Images/icons/error.png">
                                </asp:ValidatorCalloutExtender>