Examine source code of Upload Blocker

{
  "manifest_version": 2,
  "name": "Upload Blocker",
  "version": "1.2resigned1",
  "description": "Disable file upload forms on web pages",
  "applications": {
    "gecko": {
      "strict_min_version": "60.0",
      "id": "upload-blocker@clear-code.com"
    }
  },
  "permissions": [
    "<all_urls>"
  ],
  "content_scripts": [
    {
      "all_frames": true,
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content.js"
      ],
      "match_about_blank": true,
      "run_at": "document_start"
    }
  ]
}