Renaming files is unlikely to accomplish your goal, whatever it is. Perhaps you should state what you are trying to accomplish.
In any case the approach you are pursuing is not going to work. Kindle for Android downloads most books in KFX using DRM that is not currently removable.
The first post in the thread
Dealing with Kindle for PC/Mac 1.19 and KFX in calibre enumerates the methods that can be currently used to obtain Kindle ebook files in a usable format.