title: Binding a property with is not a dependency property
body: <p>I want to bind Position of media element to it model view. I know about that property not being a dependency property. So tried it this way, a code i found on the net</p>
  
  <pre><code>&lt;MediaElement Source="{Binding CurrentClip.Path, Converter={StaticResource converter}, UpdateSourceTrigger=PropertyChanged}" Stretch="Uniform" local:MediaElementHelper.Postion="{Binding CurrentClip.Postion}"
  </code></pre>
  
  <p>MediaElementHelper</p>
  
  <pre><code>class MediaElementHelper
  {
      public static readonly DependencyProperty PostionProperty =
          DependencyProperty.RegisterAttached("Position",
          typeof(bool), typeof(MediaElement),
          new FrameworkPropertyMetadata(false, PostionPropertyChanged));
  
      private static void PostionPropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
      {
          var richEditControl = obj as MediaElement;
  
          if (richEditControl != null)
          {
              richEditControl.Position = (TimeSpan)e.NewValue;
          }
      }
      public static void SetPostion(UIElement element, TimeSpan value)
      {
          element.SetValue(PostionProperty, value);
      }
      public static TimeSpan GetPostion(UIElement element)
      {
          return (TimeSpan)element.GetValue(PostionProperty);
      }
  }
  </code></pre>
  
  <p>[Error]
  A 'Binding' cannot be set on the 'SetPostion' property of type 'MediaElement'. A 'Binding' can only be set on a DependencyProperty of a DependencyObject.</p>
  
  <p>What am i doing wrong? </p>