nodejs gridfs grid.put() with identical filename overwrites file

When I am writing a file into GridFS using grid.put(), which has the same filename as a file stored before, the first file is going to be overwritten. Is it actually true that the same filename can only exist once in the database or am I doing something wrong?

My code looks like this:

var mongo = require('mongodb'),
  Server = mongo.Server,
  Db = mongo.Db,
  Grid = mongo.Grid;
  server = new Server('localhost', 27017, {auto_reconnect: true});
  db = new Db('mydb', server);

db.open(function(err, db) {
  var buffer = new Buffer("This is the first sample text");
  grid.put(buffer, {metadata:{}, filename: "test.txt", content_type: 'text'}, function(err, fileInfo) {
    buffer = new Buffer("This is the second sample text");
    // now this overwrites the first one...
    grid.put(buffer, {metadata:{}, filename: "test.txt", content_type: 'text'}, function(err, fileInfo) {
    });
  });
});

I thought that the file is specified unique by the ._id ObjectId and not by the filename. Am I wrong?

Thanks for your help!