Hi Vaibhav,

The best document to upload as proof of Indian nationality is your birth certificate, as it proves that both your parents are Indian, hence making you an Indian as well. If one of your parents is a foreigner, you will require the birth certificate of your parent who is an Indian as well to prove one of your parents is an Indian. You can also submit documents such as passport, voter ID, etc, but your birth certificate is the best document to prove your Indian nationality. 

Cheers.