If you try to mount an ISO file (like a Linux distribution) on MacOS and you get that weird error saying “No mountable file systems“:

Then, you can easily find a workaround by using CLI.

% mkdir /mnt/iso
% hdiutil attach -nomount FILENAME.iso
/dev/disk2              FDisk_partition_scheme          
/dev/disk2s2            0xEF
% mount -t cd9660 /dev/disk2 /mnt/iso

And now, the content of your iso file is available in /mnt/iso directory.

To unmount the ISO file, just type:

% umount /mnt/iso