Hi Moumou38,
Have you tried setting this through the project settings window in the Editor (Edit → Project Settings → Engine → General Settings → Default Classes)? This is the preferred method for changing these settings, rather than by editing the .ini file (making the change in the Editor actually updates the .ini file for you).
Which version of 4.7 are you using, and did you build the Engine from source code or are you using the binary version installed by the Engine? Which version of the Engine were you using when this was working for you?