DM500-S/C/T, DM56x0, and DM7000 use *.img image files,
or Flaswizard backup images with *.fws
*.nfi image files are only made for the Dreamboxes that has a secondstage bootloader.
That would be all the Dreamboxes that can be flashed through web browser:
DM500+
DM500HD
DM600PVR
DM800HD
DM800HDse
DM7020S(i)
DM7025
DM7020HD
DM8000HD