That's not the way to attach a trigger for file upload.
Try this:
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server"> </asp:ScriptManager>
<asp:UpdatePanel ID="updPanel" runat="server" UpdateMode="Conditional">
<Triggers>
<asp:PostBackTrigger ControlID="btnUpload" />
</Triggers>
<ContentTemplate>
<asp:FileUpload ID="fupload" runat="server" /><br />
<asp:Button ID="btnUpload" runat="server" Text="Upload" onclick="btnUpload_Click" />
</ContentTemplate>
</asp:UpdatePanel>
</div>