Examine source code of MB - File size Checker

{
  "manifest_version": 2,
  "name": "MB - File size Checker",
  "version": "1.1",
  "description": "Adds Section to the Context menu that displays size of targeted file",
  "icons": {
    "64": "icon.png"
  },
  "author": "Creepler13",
  "permissions": [
    "contextMenus"
  ],
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ]
    }
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "{76234758-e24f-4b95-b41a-d7df5a2e0972}"
    }
  }
}