Depending your architecture, simplest thing I can think of is offering this download from Traditional Web module, where you can set the mime type for download. Other approach would be to offer this content as REST api, where you can set Content-Type header to whatever you wish.
I think there's very little you can do with mime types for reactive pages (text/html) because it's single page app and that download of yours is served by javascript on that screen instead of the whole page being your download content.
Here's a screenshot from similar reactive demo page:

br,
-Mikko(N)
edit: fixed a typo