Hello Sodi We,
You can access the UploadButton element (named "UploadButton") from the Controltemplate of the RadButton and disable it. A possible approach would be to use the ChildrenOfType() method to find all buttons in the control template of the RadUpload. And then disable the one named "UploadButton". However, you should keep in mind that you will have to use a Dispatcher to delay the disabling of the button a bit in order to implement this scenario.
I prepared a sample project illustrating this approach. Give it a try and let me know if it works for you.
All the best,
the Telerik team
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