You would need to look at a variety of browsers. I only quoted Firefox claim. One maybe-not-critical point is that browsers might use the internal display of file extensions that they know how to display, such as
I suppose there is also a chance that links (especially for unknown extensions) could use a viewer that’s a different web browser on that system (e.g. if association was set), which would maybe look a little strange.
Although I haven’t tested with an HTML link, testing from the address bar with a
file:// link to a local file, then looking around with Process Explorer found both Firefox and Chrome seemingly invoking the external program through Explorer (it’s the parent process of the viewer, and one can see the exact invocation line).
Both programs started the viewer with the original file path, but Chrome downloaded the file first (clutter…) reminding me that a cleanup scheme of some sort would be preferable, if a preview scheme is automated.
Your web development skills seem pretty good, so if you would like to look at code, the HTML part of this is:
Pull requests can be submitted, and community help for any part of Duplicati helps keep the project moving.