MongoDb。GridFS。FindOneById返回null

我正在使用以下代码将图像上载到服务器:

public ActionResult AttachImage(HttpPostedFileBase file)
{
   var options = new MongoGridFSCreateOptions
   {
      Id = ObjectId.GenerateNewId().ToString(),
      ContentType = file.ContentType
    };
    Context.Database.GridFS.Upload(file.InputStream, file.FileName, options);
    return RedirectToAction("Index");
}

并尝试获取如下文件:

public ActionResult GetImage(string id)
{
   var image = Context.Database.GridFS.FindOneById(new ObjectId(id));
   if(image == null)
   {
      return HttpNotFound();
   }
   return File(image.OpenRead(), image.ContentType);
}

上传后,我可以在数据库中看到文件,但当我尝试将其加载为

Context.Database.GridFS.FindOneById(new ObjectId(id));

我总是得到空值。你能告诉我我做错了什么吗?

public class DbContext
{
    public MongoDatabase Database;
    public DbContext()
    {
        var client = new MongoClient(Properties.Settings.Default.ConnectionString);
        var server = client.GetServer();
        Database = server.GetDatabase(Properties.Settings.Default.DatabaseName);
    }
}

mongocsharpdriver 2.5.0 mongo server 3.6