How to extract stock apk from TWRP backup? I installed TWRP recovery on my phone and backed up the system, data and boot partitions before installing CM12.1.

I would like to extract one stock apk (FM radio) from the backup on a Linux system. How do I mount the system.ext4.win file?
Rename the system.ext4.win file to system.ext4.tarand extract it.
If your
TWRP backup is not in compression method, you will directly get to the /system folder or if your TWRP backup is in compression method, you will get a file named system.ext4 which you'll have to just rename it to system.ext4.tar and extract it to /system folder