如何在Heroku上设置特定文件的权限

I wish to set a forbidden access to a file on heroku using php. I saw relating question HERE, but the code supplied isn't working for me and i couldn't get any response.

Here is my composer.json file:

{
  "require": {
    "ext-gd": "*"
  },
  "scripts": {
    "compile": [
      "chmod 600 folder/a.jpg"
    ]
  }
}

But even after this, i was still able to access www.example.com/folder/a.jpg.

Any help will be appreciated.