Mongoid doesn't use the "official" Ruby driver to talk to MongoDB and that's where Mongo::GridFileSystem
comes from. Mongoid uses Moped to talk to MongoDB and Moped doesn't know anything about GridFS.
AFAIK the usual GridFS solution is to use mongoid-grid_fs to talk to GridFS:
self.response_body = Mongoid::GridFs[gridfs_path].data
or if you have the id
instead of the path:
self.response_body = Mongoid::GridFs.get(gridfs_id).data