The code you provided works fine. However, if I wrap it in a minimal Master page with nothing more than a simple ContentPlaceholder, ONLY 50-75% of the upload is performed, then the process hangs. I tried both the MS Ajax version, and the No Ajax version of the control. Does the control work with Master Pages?
I modify your code to use the MasterPage like
<asp:Content ID="content1" runat="server" ContentPlaceHolderID="ContentPlaceHolder1">
<CuteWebUI:Uploader runat="server" ID="uploader" OnFileUploaded="file_uploaded" >
</CuteWebUI:Uploader>
</asp:Content>
Master page (no code behind):
<body>
<form id="form1" runat="server">
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
</asp:ContentPlaceHolder>
</form>
</body>