AsyncPostBack FileUpload on a ModalPopup - asp.net

Question

I am using a UpdatePanel, and my submit button is one of the triggers along with the clear button. But the problem is I have a FileUpload Control in the div. This is a modal popup, so it displays a form for a user to upload a little note. When I try to upload a file with AsyncPostBackTrigger it does nothing (which I have read about). My question is how do I not use the PostBackTrigger because I want to use the asyncpostbacktrigger because if an error occurs, then the Modal popup closes and the user doesnt know if the file was uploaded or not. What can I do?
Code:
<asp:Panel ID="addnotepanel" runat="server" style="/*display:none;*/" CssClass="addnotepanel">
<asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate>
<asp:FileUpload ID="FileUpload1" runat="server" />
<br />
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="ClrBtn" />
<asp:PostBackTrigger ControlID="SubmitBtn" />
</Triggers>
</asp:UpdatePanel>
File:
<br />
<asp:Label ID="ErrorLabel" runat="server" Visible="False"></asp:Label>
<br />
<asp:Button ID="Submitbtn" runat="server" Text="Submit"
onclick="Submitbtn_Click" />
<asp:Button ID="CnlBtn" runat="server" Text="Cancel" onclick="CnlBtn_Click" />
<asp:Button ID="ClrBtn" runat="server" onclick="ClrBtn_Click"
Text="Clear" />
</div></asp:Panel>

Answer 1

keep SubmitBtn as a PostBackTrigger but dont set it as the "OkControlID" of the modalpopupextender.
in the Submitbtn_Click server side sub, call yourModalpopupextenderID.hide() if the upload is completed so the Modal pop up closes only if there is no error.
You could use a AsyncFileUpload from the AjaxControlToolkit
Here, as an exemple some code to show how to use it :
<AjaxControlToolkit:AsyncFileUpload ID="AttachementsFileUpload"
runat="server"
OnUploadedComplete="AttachementsFileUpload_UploadedComplete"
OnClientUploadComplete="uploadComplete" />
<script type="text/javascript">
var UpdateAttachementsGridViewButton = '<%= UpdateAttachementsGridViewButton.ClientID %>';
function uploadComplete(sender, args) {
$get(UpdateAttachementsGridViewButton).click();
}
</script>
As you see, when the upload is complete, I use Javascript to trigger an click on an hidden button. Meanwhile I retrieve the file in the AttachementsFileUpload_UploadedComplete using something like this:
Dim AttachementsFileUpload As AjaxControlToolkit.AsyncFileUpload = AnnouncementFormView.FindControl("AttachementsFileUpload")
Attachements.add(e.filename, AttachementsFileUpload.FileBytes)
It's how I used it in my situation, but you will find plenty of examples on how it work