Examine source code of Never Force Download

{
  "manifest_version": 2,
  "name": "Never Force Download",
  "description": "For file types which Firefox can display (e.g. image, text, PDF), never force a download.",
  "version": "7",
  "icons": {
    "256": "never-download.png"
  },
  "background": {
    "scripts": [
      "never-download.js"
    ]
  },
  "permissions": [
    "<all_urls>",
    "webRequest",
    "webRequestBlocking"
  ]
}