Here's a link about it.

I've seen downloads that have .MHT files. These are all over the map as to what they contain so I can't offer anything specific except Microsoft seems to be dropping support for it.

There should be many downloads out there containing or being just a MHT and since no one is actively converting these to something else, you have a choice to make. Ignore the download or research how to add support for it to your choice of OS.