Attached property of type list

I want to create an attached property that can be used with this syntax:

<Button>
  <Image .../>
  <ui:ToolbarItem.DisplayFilter>
    <TabItem .../>
    <TabItem .../>
    <TabItem .../>
  </ui:ToolbarItem.DisplayFilter>
</Button> 

This is my attempt at doing so:

public class ToolbarItem
{
  /// <summary>
  /// Identifies the DisplayFilter attached property. 
  /// </summary>
  public static readonly DependencyProperty DisplayFilterProperty =
    DependencyProperty.RegisterAttached(
     "DisplayFilter",
     typeof( IList ),
     typeof( ToolbarItem )
    );

  public static IList GetDisplayFilter( Control item ) {
    return (IList)item.GetValue( DisplayFilterProperty );
  }

  public static void SetDisplayFilter( Control item, IList value ) {
    item.SetValue( DisplayFilterProperty, value );
  }
}

This, however, is causing an exception at parse-time -- System.ArgumentException: TabItem is not a valid value for property 'DisplayFilter'. So how do I configure my attached property so that I can use the desired XAML syntax?