I have already answered your support ticket on the subject. For convenience I will paste my answer here as well.
Since RadControls for ASP.NET AJAX Q2 2010 we have made few changes to the FileBrowserContentProvider abstract class, but most of them (except one) are backward compatible and if you implement CheckReadPermission() method to fulfill the requirements of your scenario the content provider should be working as expected.
Nevertheless, to be able to provide more to the point answer to your specific case we will need to examine and debug the custom content provider. Could you please provide a sample fully runnable project so we can test it locally?
All the best,
the Telerik team
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