When uploading files with mongofiles, the following index is created in MongoDB 3.0:

{ "v" : 1, "unique" : true, "key" : { "files_id" : 1, "n" : 1 }, "name" : "files_id_1_n_1", "ns" : "test.fs.chunks" }

In 2.6, the index is missing the unique attribute:

{ "v" : 1, "key" : { "files_id" : 1, "n" : 1 }, "name" : "files_id_1_n_1", "ns" : "test.fs.chunks" }

This causes problems with drivers that automatically call ensureIndex with the unique index and then fail because the index exists but with different options.