asp.net FileUpload doesn't work properly

Question

I have asp:FileUpload , two asp:Button , and GridView in UpldatePanel .

<asp:FileUpload runat="server" ID="fileExcelUpload" />

<asp:Button  ID="btnShow" runat="server" Width="80px" OnClick="btnShow_Clicked"
                        AutoPostBack="true" Text="Save"/>
<asp:Button  ID="btnImport" runat="server" Width="80px" OnClick="btnImport_Clicked"
                        AutoPostBack="true" Text="Save"/>


<asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional">
    <ContentTemplate>
.........GridView...........
</ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="btnShow" EventName="Click" />
        <asp:AsyncPostBackTrigger ControlID="btnImport" EventName="Click" />
    </Triggers>
</asp:UpdatePanel>

Firstly , I retrieve some data from database and bind to gridView using btnShow .It works perfectly.
But when I upload Excel file with asp:FileUpload and click btnImport ,

   protected void btnImport_Clicked(object sender, EventArgs e)
    {
        if (fileUExcelUpload.HasFile)
            { 
            .....
            }
    }

the boolean value fileUExcelUpload.HasFile return False .
But when I remove <asp:AsyncPostBackTrigger ControlID="btnImport" EventName="Click" /> from UpldatePanel's triggers , it return True .

(The reason why I add btnImport 's Click event to UpdatePanel's Triggers is , I want to persist Uploaded File of asp:FileUpload after postback . )
Is there any right way to do it ? Kindly help me Please :) Thanks !

Answer 1

Add this line in the

<Triggers> </Triggers>

<asp:PostBackTrigger ControlID="btnUpload" />

here btnUpload is the id of the fileupload control