[tomsem]

So, I just had to try this:
- upload hybrid file
- download to my Kindle (kf8 enabled), rename to .mobi so Send To Kindle would accept it
- upload the downloaded file
result:
PD service rejected it. So yes, it looks like PD really needs to see a mobi segment. Which is fine, I want to be able to read everything with Kindle for iOS, which does not support KF8 reflowable yet.

As for calibre users, they'll have to take it up with Kovid and company. If you are side loading azw3, you definitely want to use the minimal file (just kf8) to mimic what Amazon will send. But if you want to send it to PDS, then you need a hybrid file. I don't know if calibre is set up to create hybrid files as part of its kf8 conversion (it appears not), but if not, they certainly can add that feature. But they probably also have to change the extension to .mobi before emailing it. Since email is more limited (file attachment size restrictions) they might also want to consider adding an option to launch Send To Kindle to send the file instead, as you can target multiple devices and choose whether to archive or not.

I fixed the font size issue by running the 'problem' file through kindlestrip and uploading it again (it did have source archive originally). To verify that it was in fact something PDS broke, I uploaded the original again, and again, the font size increased. Pretty odd, but good to know there's a workaround.