Thanks @hmhealey. I'm using version 3.6.0 (3.6.2) Team version with postgres.
It seem the URL for the file changed to: /api/v3/teams/1rk493uf7przzxqw5khmiz9y5y/files/get/kzn7eh7xjiyo7cdbrettjf489e/texpnrfryfbz8pat54ohmtjcea/7so4ofq9i7rupko8pfrytkecir/nix.png
But I still got api.context.404.app_error. Is this the new API format?