Extra character '0' added to *.txt files during upload

Hope you can help me on this.
I am using version 5.1
When someone uploads a .txt file using the File upload field in my form, a "0" gets written in the file on the last line. If there are existing characters on the last line it overrights the last one with the "0".
Are you aware of this and is ther a fix?
It does not do this with any other type file that is uploads from this form element.
Thanks for any help.
Scott