why would "file" command not print out "ISO 9660" for .iso image?

Robert P. J. Day

was just handed this little puzzler and, before i dive into it,
perhaps someone has an answer.

in working WRL9/morty system, the "file" command (file_5.28.bb) is
used to determine whether a file is a valid ISO image, by running
"file" against the image file, expecting to get output like:

rday.iso: DOS/MBR boot sector ISO 9660 ... etc etc ...

and doing a tacky string comparison simply looking for the substring
"ISO" and, so far, it's worked just fine.

now, moving to an LTS19/zeus system -- same ISO image -- that shell
string comparison fails, apparently because the output from
file_5.37.bb for precisely the same ISO image file is only:

rday.iso: DOS/MBR boot sector

as in, that's the entire output, hence no "ISO" substring, hence
failed comparison.

i'm about to verify that all is as i've been told, then try to
figure out why the newer file command is dropping all the rest of that
output. (it's even possible that the developers put in a weird, custom
magic file but that's just speculation.)

does the above sound familiar to anyone? is the newer "file" command
sufficiently different as to have dropped that "ISO ..." part of the
output? and is there an option to restore it?

thanks for any hints.