Question: how to restore whatsapp chat from file manager?
Answer: Download a file manager app.
In the file manager app, navigate to sdcard/WhatsApp/Databases. ... 
Rename the backup file you want to restore from msgstore-YYYY-MM-DD.1.db.crypt12 to msgstore.db.crypt12. ... 
Uninstall and reinstall WhatsApp.
Tap RESTORE when prompted.