Brenden, That's what threw us off. There is no update button on an "Upload File" form control.
It only has a single button labelled "Choose File". Next to the uploaded file you have the edit pencil and the delete "x" but there is no update button. If you click the 'X' and delete the file then upload another one it gets a new GUID. If, however, you simply click the "Choose File" button and select the updated file and click "Save" it updates the attached file without changing the GUID. But this functionality is not intuitive.