Hi Pik Lee,
The problem is that your RadUpload is nested withing another control. This way you can 1) either put a hidden field on your page and on ItemDataBound server event pass the client id of the RadUpload control as value of the hidden field. Probably not applicable in your case.
2) Instead of having an inline template you can create a modal pop-up for the edit template and put the RadUpload in the markup of the modal popup page. This way the RadUpload1.ClientID can be used. A sample can be found here
3) The third option would be to make your custom control inherit from IScriptControl, then on edit of the control you will be able to pass the ClientId of the RadUpload and use it. Sample script control can be found here
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