Examine source code of Typewriter

{
  "update_url": "https://clients2.google.com/service/update2/crx",
  "manifest_version": 2,
  "name": "Typewriter",
  "description": "Plays typewriter sounds when you type on any web site.",
  "version": "1.1",
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "TypeWriter.js"
      ]
    }
  ],
  "background": {
    "page": "background.html"
  },
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "permissions": [
    "storage"
  ],
  "icons": {
    "48": "icon48.png",
    "128": "icon128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{85314dbe-0f65-4a11-8c0c-1802d1925a6f}"
    }
  }
}