One cannot paste when the AutomaticallyLoadFormatProviders property is false. Even if you register the format providers manually: 

public MainWindow()
{
    DocumentFormatProvidersManager.AutomaticallyLoadFormatProviders = false;
    DocumentFormatProvidersManager.RegisterFormatProvider(new HtmlFormatProvider());
    DocumentFormatProvidersManager.RegisterFormatProvider(new RtfFormatProvider());
    DocumentFormatProvidersManager.RegisterFormatProvider(new DocxFormatProvider());
    DocumentFormatProvidersManager.RegisterFormatProvider(new Telerik.Windows.Documents.FormatProviders.Xaml.XamlFormatProvider());
    InitializeComponent();
}