FileUpload.HasFile一直返回fasle

<asp:FileUpload ID="ImportFile" runat="server" />
<asp:Button ID="ImportButton" runat="server" Text="确定"       OnClick="ImportButton_Click" />

if (ImportFile.HasFile)
{
    string fileName = ImportFile.FileName;
    string fileExtension =System.IO.Path.GetExtension(fileName);
}

<asp:UpdatePanel runat="server">
    <ContentTemplate>
        <asp:FileUpload ID="ImportFile" runat="server" />
        <asp:Button ID="ImportButton" runat="server" Text="确定" OnClick="ImportButton_Click" />
    </ContentTemplate>
    <Triggers>
        <asp:PostBackTrigger ControlID="ImportButton" />
    </Triggers>
</asp:UpdatePanel>