In order to pop-up a message from the code behind you need to register client script to the page using the approach given in the following KB article:
Calling radalert from codebehind (all versions of RadWindow)
Regarding the opened files, RadFileExplorer fires a client event OnClientFileOpen
and you can use the following approach to check the file extension:
function OnClientFileOpen(sender, args)
var item = args.get_item();
alert("The selected item's extension is: " + item.get_extension());
alert("The selected item is a folder");
//cancles the event
I hope this helps.
the Telerik team
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