[REQUIRED] Environment info

firebase-tools:
9.2.0

Platform:
macOS

[REQUIRED] Test case

1. Unzip, and run firebase emulators:start --only hosting --project your-project-id from within the directory
2. Load http://localhost:5000/
3. The page will either load the firebase libraries (expected) or fail to load the libraries (actual)

firebase-issue-3041.zip

[REQUIRED] Steps to reproduce

1. Make index.html which contains:

<script src="/__/firebase/8.2.2/firebase-app.js"></script>
<script src="/__/firebase/8.2.2/firebase-analytics.js"></script>
<script src="/__/firebase/init.js"></script>

2. In your firebase.json config, add the following header:

{
    "key": "X-Content-Type-Options",
    "value": "nosniff"
}

3. Start the hosting emulator:

firebase emulators:start --only hosting

4. Inspect the HTTP response headers for those .js files

[REQUIRED] Expected behavior

The .js files return a Content-Type: application/javascript response header

[REQUIRED] Actual behavior

The .js files do not return any Content-Type header.

This is an issue when using the X-Content-Type-Options: nosniff header, since the browser will refuse to infer a Content Type, thereby not executing the firebase scripts.

Refused to execute script from 'http://localhost:5000/__/firebase/8.2.2/firebase-analytics.js' because its MIME type ('') is not executable, and strict MIME type checking is enabled.

This does not seem to happen in production and only affects the emulator's response of these firebase scripts (/__/firebase/$version/$library.js)