By design RadFileExplorer
(and RadControls in general) supports only ClientIdMode="AutoID"
and it is highly recommended to configure it this way in order to generate correct client IDs.
This is done because the whole implementation of the RadControls is based on the client IDs which are being generated in order to find and work with them (and respectively with the child controls with such Client IDs).
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