Please define the event handler as static (or Shared in VB.NET):

private static void FileManagerDownloaded(object sender, FileManagerDownloadedEventArgs e)

Actually, the “static” requirement is specific to FileManager events because it has to maintain state between requests. After FileManager is rendered, your host page is rendered and already finished executing so the context is lost (Page class is probably disposed). So when you click “Download” button you are no longer in the same request, i.e. in the same context of your host page. You are in a new request (http handler). That’s why you need static events to be reliable. FileManager is a complex control so you cannot simply have “postback” like a simple button control.