Hi Jan Michael,
Currently there is no property that can be used to easily do this. It is a good suggestion though and we will consider it for the next versions.
As a workaround, you can access the "Browse" button of the RadUpload
and disable it like so:
|var firstChild = VisualTreeHelper.GetChild(uploadControl, 0) as FrameworkElement;
|var browseButton = firstChild.FindName("BrowseButton") as Button;
|browseButton.IsEnabled = false;
Please note that this relies on the name of the template part and it may change in future versions (although it is unlikely) so this may be an unexpected breaking change.
the Telerik team