Examine source code of Box Critters Multi-Item Mod

{
  "manifest_version": 2,
  "name": "BC-MultiItem",
  "version": "1.2",
  "description": "Mod for box critters. Lets you change item types in-game.",
  "icons": {
    "48": "icons/multi-item-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://boxcritters.com/*"
      ],
      "js": [
        "multi-item.js"
      ]
    }
  ]
}