Grid.exist incorreclty returns found=false when searching based on _id

[curtw]

Using Grid.exist with options like {_id : "ABC123"} will always return found=false even if that _id exists in the fs.files colleciton. In contrast, if options is like {filename: "cat1.png"} then it works fine. This is with version gridfs-stream version 1.1.1.

[curtw]

Code to reproduce....

var queryOptions = {
    //*** NOTE: choose one, comment other.  Works with filename.  Fails with _id.
    //filename: 'my_file.txt'
    _id: 'ABC123'
}

var createOptions = {
    filename: 'my_file.txt',
    _id: 'ABC123'
};

var mongo = require('mongodb'),
    Grid = require('gridfs-stream'),
    stream = require('stream');

var db = new mongo.Db('issue85', new mongo.Server("127.0.0.1", 27017));

db.open(function (err) {
    if (err) {
        console.log('An error occurred!', err);
        throw err;
    }

    //drop database before test
    db.dropDatabase();

    var gfs = Grid(db, mongo);
    var writestream = gfs.createWriteStream(createOptions);

    writestream.on('close',function(){
        gfs.exist(queryOptions,function(err, found){
            if (found) {
                console.log('Found file.  It says... ');
                var readstream = gfs.createReadStream(queryOptions);
                readstream.pipe(process.stdout);
            }
            else {
                console.log('Sorry, file not found.');
            }
        });
    });

    var bufferStream = new stream.PassThrough();
    bufferStream.end(new Buffer('Hello World.'));
    bufferStream.pipe(writestream);
})

[oskr]

Same here, I tried with a valid ObjectId value as _id and works, but when try using md5 as _id, exists is always false