Examine source code of e621 full link

{
  "manifest_version": 2,
  "name": "e621 full link",
  "version": "1.2resigned1",
  "description": "Adds a clickable link for direct full picture for thumbnails on e621.net.",
  "content_scripts": [
    {
      "matches": [
        "https://e621.net/post*"
      ],
      "js": [
        "fulllink.js"
      ]
    }
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{5a5889a8-e1ee-4919-afa3-cfe7ebd89448}"
    }
  }
}