The mgo driver attempts to create an index {files_id: 1, n: 1} on the sharded_files_id_n.fs.chunks collection with unique=true, per the GridFS specification. However, until 162b080, the mgo driver used to create an index with unique=false.

The sharded_files_id_n.fs.chunks collection is being sharded without specifying that the shard key should impose a uniqueness constraint, so mongos ends up implicitly create an index {files_id: 1, n: 1} on the sharded_files_id_n.fs.chunks collection with unique=false. This causes the error "Index with name: files_id_1_n_1 already exists with different options" to be returned to the mongofiles tool.