I think I found a bug when using uploader in a container/layout that uses the mixin webix.IdSpace.
Please see here: http://webix.com/snippet/08153899
The problem is in the link_setter() method of webix.ui.uploader. It uses webix.$$() to get a reference to the list but webix.$$() cannot find it because the form is mixed in with webix.IdSpace.