I have a page that I am working on that I'm linking multiple user controls to. The user control contains 3 buttons, an attach, clear and view button. When a user clicks on any control on the page, the resulting information is "dumped" into the last visible control on the page.

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="Default" MasterPageFile="DefaultPage.master" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajaxToolkit" %>
<%@ Register tagName="FileHandler" src="FileHandling.ascx" tagPrefix="ucFile" %>
<asp:Content ID="Content1" ContentPlaceHolderID="Main" Runat="Server">
<asp:UpdatePanel ID="upPanel" UpdateMode="Conditional" runat="server">
<ContentTemplate>
<table>
<tr>
<td>
<ucFile:FileHandler ID="fFile1" runat="server" />
</td>
<td>
<ucFile:FileHandler ID="fFile2" runat="server" />
</td>
</tr>
</table>
</ContentTemplate>
</asp:UpdatePanel>
</asp:Content>

All file handling and processing is handled within the control, with an event when the upload to the file server is complete via a file name that was generated. When either button is clicked, the file name is always stored internal to the control in the last control's text box. Control code:

<table style="width: 50%;">
    <tr style="white-space: nowrap;">
        <td style="width: 1%;">
            <asp:Label runat="server" ID="lblFile" />
        </td>
        <td style="width: 20%;">
           <asp:TextBox ID="txtFile" CssClass="backColor" runat="server" OnTextChanged="FileInformationChanged" />
        </td>
        <td style="width: 1%">
            <%--<asp:Button runat="server" ID="btnUpload" CssClass="btn" Text="Attach"  OnClick="UploadFile"/>--%>
            <input type="button" id="btnUpload" class="btn" tabindex="30" value="Attach" onclick="SetupUpload();" />
        </td>
        <td style="width: 1%">
            <%--<asp:Button runat="server" ID="btnClear" Text="Clear" CssClass="btn" OnClick="ClearTextValue"/>--%>
            <input type="button" id="btnClearFile" class="btn" value="Clear" onclick="document.getElementById('<%=txtFile.ClientID%>').value = '';document.getElementById('<%=hfFile.ClientID%>').value = '';" />
        </td>
        <td style="width: 1%">
            <a href="#here" onclick="ViewLink(document.getElementById('<%=hfFile.ClientID%>').value, '')">View</a>
        </td>
        <td style="width: 1%">
            <asp:HiddenField ID="hfFile" runat="server" />
        </td>
    </tr>
</table>
<script type="text/javascript">
    var ItemPath = "";
    function SetupUpload(File) {
        ItemPath = File;
        VersionAttach('<%=UploadPath%>', 'true');
    }
    function UploadComplete(File) {
        document.getElementById('<%=txtFile.ClientID%>').value =  File.substring(File.lastIndexOf("/") + 1);
        document.getElementById('<%=hfFile.ClientID%>').value = File;
        alert('<%=txtFile.Text %>');
        alert('<%=ClientID %>')
    }
    function ViewLink(File, Alert) {
        if (File != "") {
            if (File.indexOf("../data/") != -1) {
                window.open(File, '_blank');
            }
            else {
                window.open('../data/<%=UploadPath%>/' + File, '_blank');
            }
        }
        else if (Alert == "") {
            alert('No file has been uploaded for this field.');
        }
    }
</script>