require a file type field in a profile

I don't seem to have what it takes to figure out how to successfully require a file type field in a profile:

• I created a custom field with 'field type = file' and 'required = Y'.
• I created a profile to be 'used for' both 'profile' and 'search results'.
• I added the file field to the profile and, again, set 'required = Y'.
• I had no trouble uploading a file with the profile, however,
• when I try to edit the contact with the profile, the input for the field is empty (the profile does correctly display the link to the file and to delete it). That doesn't bother me, but
• if I 'Submit' the profile without deleting the old and uploading a new file, I get a 'is a required field' error.

That's a simple example that would be useless in practice, but I'd like users to be able to edit other fields in the profile without getting this error.