modal jQuery pop-up and postback

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Javascript

jQuery

ASP.NET4

, +

Hi Experts,
I have a Modal jQuery pop-up opens from c# code. My pop-up has an updatePanel, I had a problem with postback when clicking asp:Button or check a checkbox, but i solved it by moving the dialogue back into the form.
Now I have added a FileUpload control to the updatePanel. and the control loses its file once i check checkbox or clicking the button, so I had to add a trigger to my updatePanel. But with the trigger the pop-up returned to close again on button click.
Here is my javaScript for the modal Pop-up:

<!-- popup javascript and links -->
   <link rel="stylesheet" href="//code.jquery.com/ui/1.11.4/themes/smoothness/jquery-ui.css"/>
  <script src="//code.jquery.com/jquery-1.10.2.js"></script>
  <script src="//code.jquery.com/ui/1.11.4/jquery-ui.js"></script>

    <script type="text/javascript">
        function showmodalpopup() {
            var dlg = jQuery("#popupdiv").dialog({
                title: "Edit IC data",
                width: 1000,
                height: 500,
                modal: true
                //buttons: {
                //    Close: function () {
                //        $(this).dialog('close');
                //    }
                //}
            });
            dlg.parent().appendTo(jQuery("form:first")); //to include modal popup contents in the form to get their values and prevent closing on postback
        };
        </script>

Here is how I open it from code behind (calling it from ListView ItemCommand):

if (e.CommandName == "editRow")
            {
                ScriptManager.RegisterStartupScript(this, GetType(), "Show Modal Popup", "showmodalpopup();", true); //to show the iQuery popup with the required info.
            }

My pop-up div:

<div id="popupdiv" title="Basic modal dialog" style="display: none">
        <div class="span12" style="text-align:left;margin-removed0px;"> 
                    <table class="table table-hover table-striped table-condensed" style="margin-removed 0px;">
                        <tr style="background-color: #10466E;">
                            <th>Function</th>
                            <th>Color</th>
                            <th>Polarity</th>
                            <th>Location</th>
                            <th>Files</th>
                            <th>Notes</th>
                        </tr>
                        <tr>
                            <td><asp:TextBox runat="server" ID="txtFunction" Enabled="false"></asp:TextBox></td>
                            <td><asp:TextBox runat="server" ID="txtColor"></asp:TextBox></td>
                            <td><asp:TextBox runat="server" ID="txtPolarity"></asp:TextBox></td>
                            <td><textarea runat="server" id="txtLocation"></textarea></td>
                            <td>
                                <asp:FileUpload runat="server" ID="picFile" /><br />
                                <asp:RegularExpressionValidator ID="fileValid" runat="server" ControlToValidate="picFile" ErrorMessage="Only .png or .jpg" ValidationExpression="^(([a-zA-Z]:)|(\\{2}\w+)\$?)(\\(\w[\w].*))(.jpg|.jpeg|.png)$" Display="Dynamic" ForeColor="Red"></asp:RegularExpressionValidator>
                            </td>
                            <td><textarea runat="server" id="txtNotes"></textarea></td>
                        </tr>
                    </table>
                   
                    <div class="span6" style="text-align:left;">
	<!-- updatePanel to prevent closing on postback -->					
                <asp:UpdatePanel runat="server" ID="updatePanel1">
				<!-- trigger to retain FileUpload file when postback-->
                    <Triggers>
                        <asp:PostBackTrigger ControlID="btnUpdate" />
                    </Triggers>
                <ContentTemplate>                    
                        <asp:CheckBoxList runat="server" ID="chkVehicles" DataTextField="Vehicle" DataValueField="Id" AutoPostBack="true" OnSelectedIndexChanged="chkVehicles_SelectedIndexChanged">
                        </asp:CheckBoxList>

                   <asp:Button runat="server" ID="btnUpdate" Text="Update" CssClass="con-tbutton" OnClick="btnUpdate_Click" UseSubmitBehavior="false" /><br />
                <asp:Label runat="server" ID="lbl_Result"></asp:Label><br /><asp:Label runat="server" ID="lbl_UpdateMessage"></asp:Label>     
                </ContentTemplate>
            </asp:UpdatePanel>
            </div>
        </div>
    </div>

Everything works very well, except that the modal pop-up closes once I click the update Button.

Please advise
thanks in advance