MongoGridFSStream.Dispose() behaves strangely and (from my point of view) inconsistently:
1. It always tries to update file's MD5 and length, even if file was not changed (and even if stream was opened only for reading)
2. It has the UpdateMD5 property that, when set to false, leads to file's MD5 being updated to null (we actually botched MD5 for all of our files with this, hehe)