ASP.NET AJAX - Why is FileUpload1.PostedFile.FileName always NULL

Hi

Im using VC#.NET 2.0 and AJAX

Im trying  to display an animated gif while Im uploading a file to my server. However the string "File1" in the (button3_click event below) is always null. This has something to do with using AJAX. If I dont use AJAX it works fine, "File1" contains the name of the file to be uploaded. Ive also inluded the ASP.NET code. Any ideas?

protected void Button3_Click(object sender, EventArgs e)
 {
       string File1 = FileUpload1.PostedFile.FileName;
 
}

ASP.NET code:

<asp:ScriptManager ID="ScriptManager1" runat="server" />
        <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
            <ContentTemplate>
                           
                 <asp:Button ID="Button3" runat="server" Style="z-index: 113; left: 8px; position: absolute;
            top: 108px" Text="Button" OnClick="Button3_Click" />
                <asp:FileUpload ID="FileUpload1" runat="server" Style="z-index: 101; left: 74px;
            position: absolute; top: 438px" />
               
            </ContentTemplate>
        </asp:UpdatePanel>
       
        <asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
            <ProgressTemplate>
                <asp:Image ID="Image1" runat="server" ImageUrl="~/Graphics/CAFQW3B9.gif" Style="z-index: 100;
                    left: 76px; position: absolute; top: 488px" />
               
                     </ProgressTemplate>
        </asp:UpdateProgress>