Black dot lower right corner of file icons

Um ... yeah, so I searched quite a bit for this and in particular, felt that https://www.jetbrains.com/help/idea/symbols.html should have definitely had the answer. I guess I would consider this to be incomplete documentation.

Does it indicate an sbt file? Both black dotted files are .sbt files. If so, I can not find any documentation on this. Plus, why would you use such an ominous color and then not document it?

Oh well.