I already answered you support ticket on the subject. For convenience I will post my answer here as well.
If you want to preselect a single item in RadFileExplorer, you can user its server side property InitialPath to achieve this. Please see the following live demo for more information:
The selectFileList method of the RadFileExplorer's FileList client-object accepts an array of string providing the names of the files to be selected. Please note that first you need to navigate to the folder containing these files, you can do that either using InitialPath property or calling loadFolder(path) client-side method of RadFileExplorer.
More detailed information regarding RadFileExplorer's client-side and server-side members is available in our online documentation.
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