FExIO

for changing the extension of all files in specific directory that was stored in android file system storage

the app. needs special permission for access to the all files in storage
from setting
Privacy
And then Permission manager

make sure that you are see the video of using to granted the permission and best using
this app for advance people have good knowledge in information technology

this app. open sources under MPL-2.0 license
https://github.com/tabarakMohammed/FExIO