Send Mass Mail + HTML inserter for Gmail

manifest.json

{
  "update_url": "https://clients2.google.com/service/update2/crx",
  "manifest_version": 3,
  "name": "Send Mass Mail + HTML inserter for Gmail",
  "description": "A Gmail Extension",
  "version": "2.2.1",
  "icons": {
    "16": "assets/icons/mailbrother_16.png",
    "32": "assets/icons/mailbrother_32.png",
    "48": "assets/icons/mailbrother_48.png",
    "64": "assets/icons/mailbrother_64.png",
    "128": "assets/icons/mailbrother_128.png"
  },
  "host_permissions": [
    "https://mail.google.com/*"
  ],
  "content_scripts": [
    {
      "matches": [
        "*://mail.google.com/*"
      ],
      "js": [
        "inject.js"
      ],
      "run_at": "document_start"
    }
  ],
  "web_accessible_resources": [
    {
      "resources": [
        "main.js",
        "*.css",
        "dependency.*",
        "assets/js/gsi_client.js"
      ],
      "matches": [
        "*://mail.google.com/*"
      ]
    }
  ]
}