ASP.NET-带回发触发器的文件上载

我有一个UpdatePanel,它有一个上载控件和一个按钮来上载。按钮被设置为触发器,但按钮的事件处理程序无法在第一次回发时执行。

<asp:UpdatePanel ID="updPages" runat="server" UpdateMode="Conditional">
    <ContentTemplate>
        <div class="tabs">
            <ul>
                <li><asp:LinkButton ID="lnkContentsPages" runat="server" OnClick="updateContents" CommandArgument="pages">Pages</asp:LinkButton></li>
        <%-- these tabs change the ActiveViewIndex to show a different UserControl --%>
                <li><asp:LinkButton ID="lnkContentsImages" runat="server" OnClick="updateContents" CommandArgument="images">Images</asp:LinkButton></li>
            </ul>
            <div class="tabbedContent">
                <asp:MultiView runat="server" ID="mltContentsInner" ActiveViewIndex="0">
                    <asp:View ID="viwContentsImages" runat="server">
                        // ajax usercontrol for a list of images - works fine with ajax
                        <fieldset>
                            <legend>Upload New</legend>
                            <div class="formRow">
                                <asp:Label ID="lblFile" runat="server" Text="Filename" AssociatedControlID="uplFile" />
                                <asp:FileUpload ID="uplFile" runat="server" />
                            </div>
                            <div class="formRow">
                                <asp:Label ID="lblImageDescription" runat="server" Text="Description" AssociatedControlID="txtImageDescription" />
                                <asp:TextBox runat="server" ID="txtImageDescription" />
                            </div>
                            <asp:Button ID="btnUpload" runat="server" Text="Upload" CssClass="c3button btnUpload" CausesValidation="false" OnClick="btnUpload_Click" />
                        </fieldset>
                    </asp:View>
                    <asp:View ID="viwContentsPages" runat="server">
                        // ajax usercontrol - works fine with ajax
                    </asp:View>
                </asp:MultiView>
            </div>
        </div>
    </ContentTemplate>
    <Triggers>
        <asp:PostBackTrigger ControlID="btnUpload" />
    </Triggers>
</asp:UpdatePanel>

这个按钮在第二次及以后的时间里都能正常工作,只是第一次不成功。有什么原因吗?