chromium/chrome/test/data/web_app_file_handling/icons_app_manifest.json

{
  "short_name": "file editor",
  "name": "super duper file editor",
  "icons": [
    {
      "src": "icon128.png",
      "sizes": "144x144"
    }
  ],
  "start_url": "/icons_app.html",
  "display": "standalone",
  "scope": "/",
  "file_handlers": [
    {
      "action": "/icons_app.html",
      "name": "Plain Text",
      "accept": {
        "text/plain": [".txt", ".md", ".csv", ".text"]
      },
      "icons": [
        {
          "src": "icon20.png",
          "sizes": "16x16"
        }
      ]
    }
  ]
}