> Update: It looks like Amazon is converting the uploaded EPUB to KF8 (AZW3). So, it looks like they might not support EPUB natively.
So I'm not holding my breath. Which is not to say Amazon couldn't add native EPUB support. KF8/AZW3[1] (Amazon's proprietary ebook format) is basically EPUB in a MOBI shell. But they're moving away from KF8 to KFX[2] for books purchased from the Amazon store.
[1] https://wiki.mobileread.com/wiki/KF8 [2] https://wiki.mobileread.com/wiki/KFX