file upload control

hi

i use these code to uploading image

<asp:UpdatePanel ID="Upbdt" runat="server">
    
    <ContentTemplate>
    <asp:ModalPopupExtender DropShadow="true" ID="ModalPopupExtender"  
            PopupControlID="PnPopup" runat="server" TargetControlID="BtnPopup" 
             >
    </asp:ModalPopupExtender>
    <asp:Button ID="BtnPopup" OnClick="BtnPopup_Click" runat="server" Text="upload" CssClass="DPC9_Img1" BorderStyle="Solid" BorderWidth="0px" BorderColor="White" />
    <asp:Panel ID="PnPopup" runat="server" BackColor="White"  
            CssClass="panel">
        <asp:ImageButton ID="ImageButton1" runat="server" ImageUrl="~/image/close.png" />
   <br />
        <asp:FileUpload ID="AsyncFileUpload1" runat="server" 
            ToolTip="click" />
   
    <br />  
 
     <asp:Button ID="BtnUpload" OnClick="BtnUpload_Click"  runat="server" Text="load" />
     <asp:Label ID="lblMessage" runat="server" Text="  "></asp:Label>
      <asp:Label ID="Label20" runat="server" Text="Label"></asp:Label>
    </asp:Panel>
    </ContentTemplate>   
    <Triggers>
    <asp:PostBackTrigger ControlID="BtnUpload" />
    
    </Triggers>
    
    </asp:UpdatePanel>

behind code

protected void BtnUpload_Click(object sender, EventArgs e)
    {

        string path = Server.MapPath(".") + "\\../Upload\\";

        string[] validext = { ".jpg", ".gif", ".png", ".rar" };
        string ext =
            System.IO.Path.GetExtension(AsyncFileUpload1.PostedFile.FileName);
        if (Array.IndexOf(validext, ext.ToLower()) < 0)
        {
            lblMessage.Text = "please enter .jpg .png .gif  format.";
            return;
        }

        //4-Get File Name
        string filename = System.IO.Path.GetFileName(AsyncFileUpload1.PostedFile.FileName);

        //5-Get File Exist and if (true)Generate New Name
        while (System.IO.File.Exists(path + "\\" + filename))
            filename = "1" + filename;

        //6-Save File to Server


        if (AsyncFileUpload1.HasFile)
        {
            try
            {
                // Set file size limit(in bytes) condition to disable the 
                // users to upload large files. Here 1024 bytes = 1 kilobyte.
                if (AsyncFileUpload1.PostedFile.ContentLength < 1050)
                {
                    // SaveAs method of PostedFile property used
                    // to save the file at specified rooted path
                    AsyncFileUpload1.PostedFile.SaveAs(Server.MapPath("~/Upload") + System.IO.Path.DirectorySeparatorChar + AsyncFileUpload1.PostedFile.FileName);

                    // success message
                    Label20.Text = "File " + AsyncFileUpload1.PostedFile.FileName + " uploaded successfully.";
                }
                else
                {
                    // file size limit exceeded
                    Label20.Text = "File size of " + Convert.ToString(AsyncFileUpload1.PostedFile.ContentLength / 1024 ) + " KB is exceeding the uploading limit.";
                }
            }
            catch (Exception ex)
            {
                // error message
                Label20.Text = "Some problem occurred while uploading the file. Please try after some time.";
            }
        }
        else
        {
            // warning message
            Label20.Text = "Please choose a file to upload.";
        }

        AsyncFileUpload1.PostedFile.SaveAs(path + filename);

        SqlCommand _cmd = new SqlCommand("Fileup1", _cn);
        _cmd.CommandType = CommandType.StoredProcedure;
        _cn.Open();
        _cmd.Parameters.AddWithValue("@image", AsyncFileUpload1.FileBytes);
        _cmd.Parameters.AddWithValue("@behcode", "8888");
        _cmd.Parameters.AddWithValue("@name", "test");
        _cmd.ExecuteNonQuery();
        _cn.Close();

here in UpdatePanel i have 2 lable :1-lable20    and   2-lblMessage   

these lable show error message when Extension and size of image are out of limit.

problem is when i.e  I upload  .rar file and click on BtnUpload button it exit from popup window and i cant't see error message i should click on upload button again  untill again show popup window after that i can see error message

 i want it doesn't exit from popup window until user enter correct format of image if they don't enter correct format they can see error on popup window

Best regards