I'm not sure if this is related to the 6.3 update, but:
- Some of my sideloaded MOBIs are no longer showing in the Doc tab
- A MOBI created with the latest Kindle Previewer 2.4 app and sideloaded to the fire is not recognized
BTW, the MOBI created with the latest Kindle Previewer app displays correctly in Kindle Previewer 2.4, Kindle for PC, and on the K3 Keyboard when sideloaded. On the Fire it is not recognized as a MOBI.