electron: Extensions don't work with file:// protocol since 9.0.0

Extracted from #23662, https://github.com/electron/electron/issues/23662#issuecomment-632344005, per https://github.com/electron/electron/issues/23662#issuecomment-637077040 (cc @nornagon).

Extensions get loaded for http:// and https:// protocols, but don’t work for file:// protocol, which is commonly used.

This also affects devtools extensions.

Appears to be an undocumented regression from 8.x.

Chrome by default behaves the same, but it has an Allow access to file URLs switch (the fileAccess flag) on extension page at chrome://extensions/, which enables the extension to run on file:// urls (which can be verified on Content scripts tab). Any hint how to do that in Electron? React-devtools works on file:// protocol in Chrome with that flag enabled, but does not work in Electron since 9.0.0, as there are no ways to toggle that setting.

Preflight Checklist

• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Issue Details

• Electron Version:
  • 9.0.0, 9.0.2
• Operating System:
  • Linux xps 5.6.14-arch1-1 #1 SMP PREEMPT Wed, 20 May 2020 20:43:19 +0000 x86_64 GNU/Linux
• Last Known Working Electron version:
  • 8.3.1

Expected Behavior

Installed extensions should load for file:// protocol automatically, or/and there should be an opt-in/out-out for that (like opt-in in Chrome).

Extension content script present in Content scripts tab.

Actual Behavior

Extensions are not loaded if the for pages requested over file:// protocol.

No extension content script present in Content scripts tab.

To Reproduce

index.js:

const { app, session, BrowserWindow } = require('electron');
const path = require('path');

(async () => {
  await app.whenReady()
  await session.defaultSession.loadExtension(path.join(__dirname, 'extensions/test'))
  const bw = new BrowserWindow({webPreferences: {sandbox: true}})
  //await bw.loadURL('about:blank')
  //await bw.loadURL('https://example.org/')
  await bw.loadURL(`file://${path.resolve('simple.html')}`)
  bw.show()
  bw.openDevTools()
})()

Put anything in simple.html.

Example extension (but you can use any other, e.g. https://github.com/electron/electron/tree/master/spec-main/fixtures/extensions/chrome-api or react-devtools)

extensions/test/main.js

console.log('extension loaded')

extensions/test/manifest.json

{
  "name": "test",
  "version": "1.0",
  "content_scripts": [
    {
      "matches": ["<all_urls>"],
      "js": ["main.js"],
      "run_at": "document_start"
    }
  ],
  "permissions": [ "file:///*", "http://*/*", "https://*/*" ],
  "manifest_version": 2
}

Screenshots

https://:

file://:

Additional Information

This breaks devtools extension and is a part (if not the main reason) behind #23662.