The best option is to use a Custom content provider for the FileExplorer
control. This is the most secure and easy way to do it. All you need to do is inherit the default content provider and define some custom logic. Please, check the following article for more information on the topic - http://www.telerik.com/help/aspnet-ajax/fileexplorer-custom-filebrowsercontentprovider.html
Hope this helps.
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