Examine source code of SmartCropper

{
  "manifest_version": 2,
  "name": "SmartCropper",
  "version": "1.0",
  "description": "Select or auto-detect the main object in images using simple contrast/contour heuristics, then crop and export PNG.",
  "icons": {},
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "smartcropper.js"
      ],
      "css": [
        "smartcropper.css"
      ],
      "run_at": "document_end"
    }
  ],
  "permissions": [
    "activeTab",
    "storage"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{82806f28-54b5-4b56-b737-3e1d173ad6e4}"
    }
  }
}