Having the attach function enabled, when you try to submit a post that do not have an attach you got a message : "File could not be uploaded." the language variable for NOT_UPLOADED
The issue is that the name is not really empty as it is supposed to be, there are some strange characters ''
As you will not see it property I will attach a file, please open it with the regular Windows Wordpad to see it
I got it by doing a print_r( $_FILES[$form_name]['name'] );