Hi Marja,
I have just answered you support ticket on the subject, for convenience will paste my answer here as well:
The control used for listing the files and folders in the FileBrowser dialogs is FileExplorer and, unfortunately, it is not possible to use a custom upload dialog in it. The main reason for that is the implementation of the Uploading functionality is very complex, connecting with setting/getting the permissions for different folders.
Kind regards,
Veselina Raykova
Telerik
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