You are absolutely right. The Additional fields
Please, accept my apologies for the misleading information.
The OnClientAdded event allows adding new fields only client-side and this is the only current option to add any additional elements to RadUpload. Therefore, adding server controls to RadUpload is not supported at this point. The better solution in your case is to create some kind of validation which can suit your scenario.
Do not hesitate to contact us again if you have other questions.
the Telerik team