In order to change the name of the files and / or folders displayed in the file explorer, you need to subclass the RadFileExplorer's content provider and override the ResolveDirectory() / ResolveRootDirectoryAsTree() and change the Name property of the FileItem / DirectoryItem that is returned by the method.
Please review the following help article for more detailed information regarding the RadFileExplorer's content provider:
Using custom FileBrowserContentProvider
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