The index ensured by mongoc_client_get_gridfs must be unique

mongoc_client_get_gridfs() returns NULL in MongoDB versions < 3.0.

Error message is:
files_id_1_n_1 already exists with different options

When uploading files (e.g. using mongofiles), MongoDB creates an index with

in MongoDB 3.0, while it is missing in lower versions.

What is the value of ensuring this index on each access anyway? MongoDB certainly creates the index. This is also a side-effect that should be documented.