Here is my simple app:

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:app="clr-namespace:WpfApplication1"
    Title="Window1" Height="300" Width="300">
    <Window.Resources>
        <Style x:Key="Test">
            <Setter Property="Button.Template">
                <Setter.Value>
                    <ControlTemplate>
                        <Border BorderBrush="Blue"
                                BorderThickness="3"
                                Background="Black"
                                CornerRadius="{Binding app:Extras.CornerRadius}"
                                >                            
                        </Border>
                        </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </Window.Resources>
    <Grid>
        <Button Height="23" 
                HorizontalAlignment="Left"
                Margin="29,26,0,0" 
                Name="button1" 
                VerticalAlignment="Top" Width="75"
                app:Extras.CornerRadius="10"
                Style="{StaticResource Test}"
                >Button</Button>
    </Grid>
</Window>

Here is my AttachedPropery:

namespace WpfApplication1
{
    public class Extras
    {
        public static readonly DependencyProperty CornerRadiusProperty = DependencyProperty.RegisterAttached(
          "CornerRadius",
          typeof(double),
          typeof(Button),
          new FrameworkPropertyMetadata(1.0d, FrameworkPropertyMetadataOptions.AffectsRender)
        );

        public static void SetCornerRadius(UIElement element, double value)
        {
            element.SetValue(CornerRadiusProperty, value);
        }
        public static double GetCornerRadius(UIElement element)
        {
            return (double)element.GetValue(CornerRadiusProperty);
        }

    }
}

CornerRadius="{Binding app:Extras.CornerRadius}" this of course doesn't work. so how can I get value from here app:Extras.CornerRadius="10"

thanks in advance!