GraphQL multi-part request spec outlines a specification for multipart form requests in GraphQL. Jayden Seric, the author of the spec, has also authored two libraries apollo-upload-client and apollo-upload-server which makes it effortless to integrate file uploads in Apollo Server.
GridFS is a simple file system abstraction on top of MongoDB. This facilitates storage of large files (beyond BSON document size limit) in mongo database.
This post outlines the minimal integration required to save the files uploaded through the GraphQL API to GridFS.