I'm not sure, so this is pure speculation, I think the http server is meant to be a file server made to share files on the same network, so every other file than index.html is handled as a download, .html or not. That would make sense IMHO.Again, I do know that -- exactly why I said MiX's http server IS a web server, however simple. The question is why renaming index.html to test.html, test.html cannot be opened, but only downloaded.
Hence, back to my initial question, can it be fixed on MiX's side ? Any other local web server I run on the device works properly.
I guess only @HootanParsa can shed some light on this.