I don’t think CD emulation is supported by the core, or any PC-98 emulator, though I would be glad to be proven wrong. The PC-88 also supported CDs but only one emulator supports them. (Not the Quasi88 in RA.)
For the PC-98 it will have to be floppy or HDD images.
You can see a list of supported file extensions by loading the core and going to the information setting, or loading the core *.info in a text editor.
It is not unusual, for preservation and future proofing, for unsupported dumps to be released into the wild.