chrome/test/data/extensions/platform_apps/launch_file_by_extension/manifest.json

{
  "name": "Platform App Launch Test",
  "manifest_version": 2,
  "version": "1",
  "file_handlers": {
    "text": {
      "extensions": [
        "txt"
      ]
    }
  },
  "app": {
    "background": {
      "scripts": ["test.js"]
    }
  },
  "permissions": [
    "fileSystem"
  ]
}