This is the v4 pattern that depends on the client-side API - getqueuecount() and startupload() on the element, plus the CuteWebUI_AjaxUploader_OnPostback() hook. None of it existed in v5 before 5.3.2: the page compiled and rendered, then failed at runtime with "getqueuecount is not a function". A .NET API diff cannot catch that, because the missing surface is JavaScript.