The ePub file type is preferred by Amazon and nearly every other retailer for publication, so ideally, you'll want to upload the ePub file, just as it was exported from Atticus, to your KDP account when publishing. If you need more guidance on publishing to KDP and the ideal file types, I'd definitely recommend checking out Kindlepreneur's articles on the subject: https://kindlepreneur.com/how-to-publish-a-book-on-amazon/
Or this in depth look at the different file types used in publishing: https://kindlepreneur.com/epub-vs-mobi-vs-pdf/