This time we got an ext4 file. 7-zip supports a lot of formats (including virtual machines disks and filesystem images) and ext4 is one of them.
In the root directory there is a file `.mediapc_backdoor_password.gz`. When you unpack it you get `.mediapc_backdoor_password` file which contains flag.

Flag: `CTF{I_kn0W_tH15_Fs}`

Original writeup (https://github.com/BOAKGP/CTF-Writeups/blob/master/Google%20CTF%202018%20Quals%20Beginners%20Quest/Firmware/README.md).