Jeg fandt selv en løsning, men tak for tippet.
Løsningen var:
<%@ Page language="c#" Codebehind="FormUpload.aspx.cs" AutoEventWireup="false" Inherits="Dk.Integrator.CatalogMaker.Web.User.FormUpload" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<HTML>
<HEAD>
<title>TestAfUpload</title>
<!--
$Archive: /Integrator/CatalogMaker/web/User/FormUpload.aspx $
$Author: Jma $
$Date: 25-02-02 17:58 $
$Revision: 3 $
-->
<script language="c#" runat="server">
</script>
<meta content="Microsoft Visual Studio 7.0" name="GENERATOR">
<meta content="C#" name="CODE_LANGUAGE">
<meta content="JavaScript" name="vs_defaultClientScript">
<meta content="
http://schemas.microsoft.com/intellisense/ie5" name="vs_targetSchema">
</HEAD>
<body MS_POSITIONING="GridLayout">
<form id="Form1" encType="multipart/form-data" runat="server">
<table cellPadding="4" width="400" bgColor="silver">
<tr>
<td vAlign="top" width="100"><span id="hgcUpSpan" runat="server"></span></td>
<td vAlign="top"><input id="hifFilename" type="file" name="filename" runat="server">
</td>
</tr>
<tr>
<td vAlign="top"></td>
<td vAlign="top"><asp:button id="btnUpload" runat="server" Text="Upload file"></asp:button></td>
</tr>
<tr>
<td vAlign="top"><span id="hgcStatusSpan" runat="server"></span></td>
<td vAlign="top"><SPAN id="hgcStatus" runat="server"> </SPAN>
</td>
</tr>
</table>
</form>
</body>
</HTML>
Med code behind i c#: