Examine source code of fetch-downloader

{
  "manifest_version": 2,
  "name": "fetch-downloader",
  "description": "download file, download image. works on link or image",
  "version": "0.0.1.4",
  "browser_specific_settings": {
    "gecko": {
      "id": "fetch-downloader@example.com"
    }
  },
  "background": {
    "page": "background.html"
  },
  "permissions": [
    "<all_urls>",
    "contextMenus",
    "notifications",
    "downloads"
  ]
}