[jhowell]

The Amazon page I linked in my previous post indicates which file formats should go into either the "Books" or "Documents" folder.

My Fire tablet is newer than yours and I don't sideload books directly so I don't know which features will or will not be available for sideloaded books.

Generally KF8 (.azw3) format will have better formatting then MOBI, but some other features, such as cover thumbnails, may possibly work better in MOBI. You will have to try it and see.

Also I think that page numbers, which require a separate .apnx file, are unlikely to work when sideloaded on a Fire tablet. Again, either try it yourself or perhaps someone with more knowledge will comment.