I think I get what your trying to do but correct me if I am wrong.
You have an update page where you have a file field but you also have, I am assuming, a hidden field to contain the value of the original file that will be the default if there is no new file uploaded, correct?
If that is the case, I would double check that when validating the file extension of your file field that you ensure that the selection can be left blank. That way it will only validate if there is an input/upload present in the field.
Happy New Year and Cheers,