Visual Studio 2015. Files not added to TFS

I'm using Visual Studio 2015 update 3 with TFS hosted at visualstudio.com. When I add a C# class file to one of my projects in Visual Studio, it's not automatically added to source control. For other projects in the same solution, C# class files are added automatically, as expected.

When I then add the file, manually, in the project with this problem, I get the following question;

I have to confirm to add the file to source control.

There is a similar question (Visual Studio 2015 new files not being added to source control automatically), associated with a "release" branch, which is not the case here (at least I think so - our TFS-project has never been branched)

We've never configured any .tfignore manually. This started to happen recently and occurs on all development machines, so it's probably not a client machine issue.

I'm clueless, so any help is highly appreciated.

UPDATE

I've discovered that the problem occurs in projects with names ending with ".Lib"

Projects:

Core (ok)
Core.Lib (files not added automatically)
Communication (Ok)
Communication.Lib (files not added automatically)

I tried to add a test project Test.Lib to my solution. This project was not added to TFS either, but project Test was.

The "lib" extension is the problem. Try renaming the project folder to something else, eg. "CoreLib".

To reproduce this behavior / bug, try the following:

Create a Test directory in a directory controlled by TFS
Create a Test.Lib subdirectory in this directory.
Add a file, test.cs, to Test.Lib. (Test\Test.Lib\test.cs)
In Visual Studio's Source Control Explorer, try adding the new Test directory to source control. You'll see test.cs on the "Excluded items" tab.

The "lib" extension is used by binary files, and these are ignored by Team Explorer by default. I haven't found an official list of filetypes anywhere, but other affected file types are dll, exe, obj and possibly others. You can include these files manually by right-clicking them and selecting "Include".