I recently transferred an epub to my Kindle Reader app, and it worked, but there was a message saying that epub capability would be removed "later this year." Make of that what you will.
When I tried emailing an epub this way, Amazon messed up the conversion. I've had good luck converting pirated epubs to MOBI using an online converter so I haven't changed my practice.
I have an older Paperwhite; my GF has a newer one. I uploaded MOBIs to both last night (via USB) - it worked on hers, and they didn’t show on mine. I had to convert them to AZW3 and reupload for them to show in the listing.
(I’ve not checked, but it’s plausible my older model has more up-to-date firmware.)
No they don't use mobi. Amazon has two formats newer than mobi, AZW3 and KFX. Both support more features. Using mobi is like saving your Word docs as RTF these days.
This is a fairly new thing and for me has worked smoothly. I can attach a bunch of DRM-free epubs to an email addressed to my Kindle account, Amazon asks for confirmation that it's me sending them, and within about five minutes I have clean, well-formatted copies of those files on my Paperwhite.
This is what I do with my base model Kindle. I have its email address saved as a contact so it is super easy to send it ePubs (or PDFs!) from my phone.
I'm sure you're aware that amazon can discontinue this at any moment. My ebook reader just reads an sd card or I can use a usb cable to transfer files. That's harder to remotely kill.
I'm aware. If they discontinue it then I'll just stick to Kindle books. That's all I'm expecting from this device anyway. For everything else I have an iPad Pro.
They also provide an email address where, if you send an email with some epubs attached, they make them magically appear on your kindle.
I was pleasantly surprised by this last week, since I got a kindle and I thought it was going to be a hassle, but nope.