title: Assign WriteConcern to mongo file system
body: <p>Colleagues,
  I'm using Mongo of v2.2 and java Mongo driver 2.9.0,</p>
  
  <p>Some business logic creates approximately 25 threads and each thread creates 150 files on GridFS. Approximately 20 files per 1000 are not return correct <code>getId()</code> so result is null. I think (correct me if I'm wrong) it is correct behavior in perspective of throughput. But I really need this id. For regular DBCollection I would set <code>WriteConcern.FSYNC_SAFE</code>, but I cannot see if exist method <code>setWriteConcern</code> for GridFS. Have you some ideas how to force files be flushed ?</p>