CDFS is shortcut for Compact Disk File System, not an actual file extension.
CDFS file extension - Compact Disc File System
What is cdfs file? How to open cdfs files?
CDFS is not actually file extension and CDFS is not file format.
CDFS is abbreviation for Compact Disc File System, which is a ISO 9660 format for optical media.
Disk images of CDFS file system are commonly stored in ISO disk images.
There is also Linux virtual file system called CDfs, but this is not actual file format as well.