The 'tree-file' indicate if the node is a leaf node. If no child nodes exist, it acts as a leaf node. If you want to make a leaf node become a folder, please add a customer icon class to this node.
{
"id":21,
"name":"Java",
"size":"",
"date":"01/13/2010",
"state":"closed",
"iconCls":"myfolder",
}
And then define this 'myfolder' style.
<style type="text/css">
.myfolder.tree-file{
background-position: -208px 0;
}
</style>