Examine source code of Don't accept WebP

{
  "manifest_version": 2,
  "name": "Disables Webp",
  "version": "1.0",
  "description": "Disables WebP",
  "icons": {
    "48": "icon.png"
  },
  "background": {
    "scripts": [
      "src/background/webpHeaders.js"
    ]
  },
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{c53e2388-2a96-4cef-b821-9ce3c9f60bf3}"
    }
  }
}