Generally this scenario is not supported by RadAsyncUpload, but you can implement such functionality only by using the File Api upload module. This is possible when you handle the onClientFileUploading event and read the file from the get_row() property as shown in this forum.
Hope this will help you.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed