Thank you for contacting us on that matter. You are right that with the current implementation you are not allowed to base your custom style on the default one.
However, we will do our best to fix this as soon as possible. As a small sign of appreciation I updated your Telerik account.
Till we deliver the fix you can include the default style in your project so that you will be able to customize the control. For your convenience I extracted the default template of the RadUpload
control along with all the resources used in it.
Please accept our apology for the inconvenience caused.
Pavel R. Pavlov
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