For healthy chubby face, all doctors suggest me to put on weight in overall body, so i should eat lots of butter cheese, chapati and french fries?

No: No , just more of a healthy balanced diet, with more calories, not just more fat.

Underweight: If you are already at bmi 18-26, then that is ideal . A balanced diet of protein, unsaturated fats, and low glycemic carbohydrates would be better in the long run. Obesity is more of a problem to your health over a long period of time. If you are underweight, then consult a nutritionist