While modifying the SCH-i909 ROM, I extracted the firmware and found weird .rfs files instead of the usual .img files. What is this?

Google, unfortunately, had no answer. Moreover, I could NOT, for the life of me, figure out how to create an .rfs file for the new firmware, until I discovered this tutorial on XDA.

So on a Linux machine, run the following to mount your .rfs image:

mkdir temp
sudo mount -o loop <filename>.rfs temp/

Use the standard umount tool on temp/ to unmount the image after you’re done!