A official notice from JEE Advanced authority has been declared that says,

"Due to COVID-19 related restrictions hindering the issuance of various certificates in time, valid category certificate(s) may be uploaded at the time of reporting after seat allocation. For details of seat allocation schedule visit JoSAA website (https://josaa.nic.in/) ."

Here is the link : https://jeeadv.ac.in/

However you should have uploaded a declaration in lieu of OBC - NCL during application.

So don't be worried of uploading certificates now. You will have enough time during counselling process. All the best.