Examine source code of Link Size

{
  "manifest_version": 2,
  "name": "Link Size",
  "description": "Adds a context menu entry for links, returning size and last modified date",
  "author": "Pruna",
  "version": "1.4",
  "icons": {
    "16": "icon.svg",
    "32": "icon.svg",
    "48": "icon.svg",
    "96": "icon.svg",
    "128": "icon.svg"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "permissions": [
    "<all_urls>",
    "contextMenus"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "linksize@pruna"
    }
  }
}