To create a GridFS collection, open the "Advanced Settings" while creating a new collection and enable the GridsFS collection there. When saving, the entered collection name is extended with ".files" and an associated chunks collection is created.
Specifying the ".files" extension directly in the collection name makes the application detect by itself that a GridFS collection shall be created.
The remaining options of the "Advanced settings" are explained here: http://docs.mongodb.org/manual/reference/method/db.createCollection/
When using this functionality you need to take care of MongoDB's naming conventions and make sure a database with the same name does not already exist on the server.