title: What is the relationship between real collections' names and those in file system
body: <p>I use MongoDB 3.0 with WiredTiger storage engine.
  When I checked my Mongo files in dbPath, I saw the names of the files with the formats as below:
  collection-0--4989330656807016483.wt
  collection-2--4989330656807016483.wt
  collection-4--4989330656807016483.wt
  .
  .
  .
  How can I know the relationship between these file names and real collections' names except the way of data size??</p>