mount - 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?

Answer

I discovered that it was not necessary to mount it. It was sufficient to extract it with tar -xf archive_name but this particular app doesn't install.