title: Angular UI Tree: A tree with two types. A node cannot be dropped into empty node
body: <p>I have an example (<a href="http://plnkr.co/edit/u3JiQHSfBJyTnCfqGauK?p=preview" rel="nofollow">http://plnkr.co/edit/u3JiQHSfBJyTnCfqGauK?p=preview</a>) where I have a <strong>angular ui tree</strong> with two different types (folder and file). In the code I'm implementing 'accept' to allow dropping items only to appropriate targets.
  First folder "folder0" is empty. And I cannot drag and drop any other folder into it.
  If a folder has subfolders then dropping works fine.</p>