Hi,

Please set the cancel button to another button, then you can hide the custom cancel button to achieve this requirement, like below.

   <CuteWebUI:UploadAttachments ID="uploader1" runat="server" CancelButtonID="btn_Cancel">
        </CuteWebUI:UploadAttachments>
        <asp:Button ID="btn_Cancel" runat="server" Style="visibility: hidden" />
 

Regards,

Ken