1. Make sure that *.cpp and *.h files don't have an icon assigned in Windows:
   
2. Start Creator.
3. Create a new Qt Widgets Application project using the wizard.
4. On the last page of the wizard, select "Git" from the "Add to version control" combo box.
5. Select "Tools" -> "Git" -> "Local Repository" -> "Commit..." from the menu.
   The files of the project will be listed, but mainwindow.h does not have an icon:
   
6. Expand the project tree in Projects view.
   mainwindow.h does not have an icon there, either:
   
7. Restart Creator.
8. Repeat steps 3 and 4.
9. Repeat step 6.
10. Repeat step 5.
    Now icons are being shown:
    

Creator should always show the right icons, no matter in which order you execute the steps.