Examine source code of Imagify

{
  "description": "Adds a sidebar offerin a file picker and drap and drop zone. When an image file is chosen the active tab's body content is replaced with file selected. See https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Examples#imagify",
  "manifest_version": 2,
  "name": "Imagify",
  "version": "2.0",
  "homepage_url": "https://github.com/mdn/webextensions-examples/tree/master/imagify",
  "permissions": [
    "tabs",
    "<all_urls>"
  ],
  "sidebar_action": {
    "default_title": "Imagify",
    "default_panel": "sidebar/sidebar.html"
  },
  "web_accessible_resources": [
    "/viewer.html"
  ]
}