To create a GridFS collection, open the "Advanced Settings" while creating a new collection. There the GridsFS collection option must be enabled. When saving, the collection will extended with ".files" on the entered name and an associated chunks collection will created.
Alternatively you can enter the extension ".files" directly in the collection name. Then, the application detects by itself, that a GridFS collection should be created.
The remaining options of the "Advanced settings" are explained here: http://docs.mongodb.org/manual/reference/method/db.createCollection/
For this functionality it is important to note that the naming conventions for MongoDB collections are complied with and the new name does not already exist in the database.