in the File Upload server behavior, you bind the default file name to the hidden form element.
When the form is posted, File Upload Creates a variable called Server File Name
If a File is uploaded, the server File Name variable is set to the name of the file that was created on the server.
If no file is uploaded, the server file name variable is given the value of the Default File name setting. Where the default file name is bound to the hidden form element, it gets that value, which should be the file name already stored in the database.