you probably know the reason for the advice not to do things like that - if you have a lack of feeling in your feet due to nerve damage (because of the diabetes) you won't know if you're overdoing the filing and could end up with sores on your feet, which (also due to diabetes) could take a long time to heal. worst case, you could end up with ulcerated feet and that would be a big problem, maybe even leading to amputation.
if you have total, normal feeling in your feet i can't see that a little gentle filing would hurt, but the best thing is as baby jane suggests, use cream specially made for hard skin on feet, it does work but you do have to use it regularly.