Examine source code of Save As PNG!

{
  "manifest_version": 2,
  "name": "SaveAsPNG",
  "version": "1.0.0",
  "description": "Right-click any image and save it as PNG.",
  "permissions": [
    "contextMenus",
    "downloads",
    "activeTab",
    "<all_urls>"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "icons": {
    "16": "icons/icon16.png",
    "48": "icons/icon48.png",
    "96": "icons/icon96.png",
    "128": "icons/icon128.png"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "saveas-png@extension",
      "strict_min_version": "58.0",
      "data_collection_permissions": {
        "required": [
          "none"
        ]
      }
    }
  }
}