To enable the AsincUpload feature of the FileBrowser dialogs you should set EnableAsyncUpload="true" in the inner tag of the respective file browser dialog. Here is how to do that for the Image Manager, e.g.
<telerik:radeditor id="RadEditor1" runat="server">
<ImageManager ViewPaths="~/Images" UploadPaths="~/Images" DeletePaths="~/Images"
This feature is supported since Q2 2012 of RadControls for ASP.NET AJAX.
the Telerik team
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