Identify the api/widget shown / 'file stack' ui element

Looking for help identifying the following ui element or 'widget' in this app, the window is a sort of 'drop zone', you drag and drop files to it, it then displays the DnD'd files as a 'file stack' which is a ui element commonly seen in the Dock. Unable to find the api to create this. Surely it exists? (A screenshot is attached)