How to serve a wbn (WebPackage/WebBundle) file from a web server?

does anyone know how to serve a web bundle so that it loads, rather than just downloading as a file?

Some disambiguation: There is a format called WebPackage (not to be confused with webpack), also called a Web Bundle. Files typically have the .wbn suffix. It contains html and js files and can be used to view websites offline. Useful for e.g. archiving websites or making websites that work well with intermittent network access. Download the file once, and you have all the assets you need for at last basic operation of the site.

The standard on how to serve a .wbn file is here: https://wicg.github.io/webpackage/draft-yasskin-wpack-bundled-exchanges.html

However when I add the required headers in the web server, the .wbn file is just downloaded. If I drag the downloaded file onto my browser (google-chrome), the file is displayed as the website it contains, so unless there is some very subtle bug in there I believe that the format of the bundle is OK.

Here is a sample request:

Request URL: http://localhost/bundle/www-signed.wbn
Request Method: GET
Status Code: 200 OK
Remote Address: [::1]:80
Referrer Policy: strict-origin-when-cross-origin

and the server response:

Accept-Ranges: bytes
Connection: keep-alive
Content-Length: 4300
Content-Type: application/webbundle <-- Required by the standard
Date: Thu, 02 Sep 2021 12:00:24 GMT
ETag: "612ef7cb-10cc"
Last-Modified: Wed, 01 Sep 2021 03:47:23 GMT
Server: nginx/1.18.0 (Ubuntu)
X-Content-Type-Options: nosniff <-- required by the standard

If anyone has this working on a website or knows how to do it, I would love to have a look.