[Quoth]

The option to upload mobi is gone a while ago. For ordinary reflowable ebooks that Amazon gives out in Mobi (KF7), azw2/KF8 or KFX format the best upload is epub2. The MS docx format is the second-best.

I just noticed that downloading a file direct to preview on a Kindle after upload processing is gone. You either preview on the Internet Web page or they suggest download of the Kindle Previewer (which has no Linux version?) and they offer an HTML download.

Except the ‘HTML Download’ is Zip file.

Previewing inside it after download revealed what looked a bit like an epub, so I renamed the <downloaded name>.zip file to <downloaded name>.epub and it opens fine in Calibre's standalone ebook viewer on Linux.

So no need to fire up the VM with a copy of windows and install Kindle Previewer?

Actually when we use an entirely new style or HTML construct we make a dual mobi and test on three different Kindles, one only does KF7. Then we do a purchase of mobi version and azw3 versions by using accounts with suitable kindles. We gave up testing KFX because if the KF8 is OK, the KFX will be too (See also epub2 vs kobo kepub; we don't test kepub).

Obviously this is a recent change and how reliable is the idea of simple rename of .zip to .epub and using an epub viewer rather than Kindle Previewer?

Does this also suggest rename of .epub to .zip and ‘Send to Kindle’ via email will work to get an azw3/KF8 version on later Kindles?

Edit:
The .zip download renamed to .epub loaded fine into the Calibre Editor and passed Book Check and Epub Check.

Title should be KDP upload, then Kindle Preview