I am having problems getting my head around grouping and sorting in xaml and hope someone can get me straightened out!

I have creaed an xml file from a tree of files and folders (just like windows explorer) that can be serveral levels deep. I have bound a TreeView control to an xml datasource and it works great! It sorts everything alphabetically but ... I would like it to sort all folders first then all files, rather than folders listed with files, as it does now.

the xml :

if you load this to a treeviw it will display the two files before the folder because they are first in alpha-order.

here is my code:

  <!-- This will contain the XML-data. -->
  <XmlDataProvider x:Key="xmlDP" XPath="*">
     <x:XData>
        <Select_Project />
     </x:XData>
  </XmlDataProvider>

  <!-- This HierarchicalDataTemplate will visualize all XML-nodes -->
  <HierarchicalDataTemplate DataType="project" ItemsSource ="{Binding}">
     <TextBlock Text="{Binding XPath=@name}" />
  </HierarchicalDataTemplate>

  <HierarchicalDataTemplate DataType="folder" ItemsSource ="{Binding}">
     <TextBlock Text="{Binding XPath=@name}" />
  </HierarchicalDataTemplate>

  <HierarchicalDataTemplate DataType="file" ItemsSource ="{Binding}">
     <TextBlock Text="{Binding XPath=@name}" />
  </HierarchicalDataTemplate>

  <CollectionViewSource x:Key="projectView" Source="{StaticResource xmlDP}">
     <CollectionViewSource.SortDescriptions>
        <!-- ADD SORT DESCRIPTION HERE -->
     </CollectionViewSource.SortDescriptions>
  </CollectionViewSource>

  <TreeView Margin="11,79.992,18,19.089" 
            Name="tvProject" 
            BorderThickness="1" FontSize="12" FontFamily="Verdana">

     <TreeViewItem ItemsSource="{Binding Source={StaticResource xmlDP}, XPath=*}"  
                   Header="Project"/>
  </TreeView>