code: function(self, chunkNumber, options, callback) {
    if (typeof options === 'function') {
      callback = options;
      options = {};
    }
  
    options = options || self.writeConcern;
    options.readPreference = self.readPreference;
    // Get the nth chunk
    self
      .chunkCollection()
      .findOne({ files_id: self.fileId, n: chunkNumber }, options, function(err, chunk) {
        if (err) return callback(err);
  
        var finalChunk = chunk == null ? {} : chunk;
        callback(null, new Chunk(self, finalChunk, self.writeConcern));
      });
  }
docs: Gets the nth chunk of this file.
  @ignore