I have just answered you support ticket on the subject, for convenience I am pasting my answer here as well:
You could achieve the desired behavior by subclassing the the FileExplorer's built-in FileSystemContentProvider where to override its StoreFile() method. More information on how to do it is available in the following help article: Using custom FileBrowserContentProvider
In the override of the StoreFile(), you could check whether a file with such a name already exists in the current folder and to rename the uploaded file in this case:
To make the above functionality work properly, you will need to ensure that the "Overwrite if file exists?" checkbox of the upload dialog is always checked. You could achieve this in the handler of the FileExplorer's OnClientLoad event in a similar way:
For your convenience I am attaching my test project, describing the approach. I hope it would be helpful for you.
Regards,
Vesi
the Telerik team
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