As expected.
Fix that - GridFS should NOT be used in Production. And the lesson is always read ALL the documentation as most of your answers are already there.
https://docs.rocket.chat/docs/recommendations-for-file-upload
By default, GridFS is used in Rocket.Chat for file storage because MongoDB offers this functionality with zero configuration. However, it is not recommended for production environments due to the high load it places on the database.
There are two migrator repos.
Note also. Though your Mongo is ‘permitted’ by Rocket.Chat it is EOL and NOT supported by anyone. You should move ASAP, and preferably to Mongo 6+ as Mongo 5 is EOL soon.