QtCreator + CMake recognizes only *.hpp and not *.h as include files

I used QtCreator + CMake to write a small C++ project.

To my astonishment QtCreator doesn't recognize *.h as C++ files until I change the extension to *.hpp. If I use *.h I get warnings like the following:

In the CMakeFiles.txt I have the following line:

set(CMAKE_CXX_STANDARD 11)

but that doesn't help.

 The another strange thing that if I press F4 to switch source/headers QtCreators says: "the ****.h" file not found, until I open it manually and then I can switch from .hpp to .cpp and back.

QtCreator 4.9.0

Ubuntu Linux 18.04

CMake 3.10.2