private void AddNode( FileInfo fi )
{
string extension = fi.Extension;
var node = new TreeNode( fi.Name );
TreeNode parentNode = testSetNavigationTree.SelectedNode;
node.Name = fi.Name;
node.Tag = fi;
SetTreeNodeImage( extension, node, parentNode );
if (parentNode != null)
parentNode.Nodes.Add( node );
else
testSetNavigationTree.Nodes.Add( node );
}