Ajax register - erasing file for some users :(

Hi
I has encountered such a problem:
(Additional field 'file') After selecting the file, and saving the profile - the file is uploaded and its name is saved in the field. After re-edit the profile file is deleted  This happens to some users, for other all right... How to solve this problem?

P.S. When extra field (text) is exist and this field is empty, this problem is occurs. If all fields are filled, everything is fine.