Example:

As far as I understand this line is a problem - https://github.com/Microsoft/vscode/blob/5755810a58ac76ffedb037647f1cd6fec406efe9/src/vs/workbench/browser/parts/views/treeView.ts#L334

If an item has Expended or Collapsed state (which basically means it's an item with childs) then we expect folder icon.

In general, I don't think we should have such convention at all, and consumers should be able to set any icon they want - in case of my extension, tree view shows some information from the file, so it makes sense for me to use file icon on the node with child elements.

CC: @sandy081