I have defined an OnUpload handler like this on the TelerikUpload:
<TelerikUpload AutoUpload="false" ... OnUpload="@OnUploadHandler" ></TelerikUpload>
It turns out that the handler is only called when I click on the 'Upload' button. If the upload fails, the retry icon is shown. However when clicking on the Retry icon, the OnUpload handler is not called.
In my case that is a problem, because I add headers and request in this event:
It does not matter if the request is to same host or cross-domain.