I am not sure if this could be achieved easily.
Can you consider the following: adding three instances of RadUpload on the page. The first one will allow only images, the second movies and the third audios. Each upload will have the Add button, so if the user wants to upload only 2 images - he/she can add one more filie input area that will allow only images to be uploaded.
I hope this is acceptable for you.
the Telerik team
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