Thank you, guys for explanations.
So if I understand correctly:
IDE in any case will copy all the files from the Files folder to the assets folder in the apk, whether they are added to the Files tab or not
i.e.
If the files are copied to the Files folder and added to the Files tab, then IDE will
1) copy the files from the Files folder to the assets folder in the apk.
2) make them lower cased automatically
3) raise warnings (Logs tab) about files missing from the Files tab
If the files are copied to the Files folder but not added to the Files tab, then IDE will
1) copy the files from the Files folder to the assets folder in the apk.
2) not make them lower cased automatically
3) not raise warnings (Logs tab) about files missing from the Files tab