Which file types are in included in the managed property IsDocument?

There is a managed property named IsDocument which returns document type files if you property search with this set to true. If I upload a file with for example an .ifc extension in my library, then this has false for IsDocument, but true for IsListItem.

Shouldn't IsDocument in theory return true for anything you put in a document library? If my custom file types are getting IsListItem=true, then any other document is also a list item per definition..or?