This is expected behavior, please note that, RadFileExplorer does not expose all the functionality of the underlying RadTreeView - the child notes are populated on demand using callbacks.
Also, the treeview is using custom templates for displaying the nodes and the only way to modify these icons is using CSS - ImageUrl property of the node cannot be used. If you need to apply different icons on the same file types following some custom logic you can use the approach from this forum thread
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