I think the short answer is: you can't. If you pre-populate the 'file' field with a value, when the form submits, the browser will search for that file on the users local hard disk - and as you don't know which files are on the users disk, this would probably cause an error.
You'll have to work around this by maybe, on your edit page, have two bits of information - maybe 'Current File: ', with information about the currently uploaded file, and then a new 'file' box with 'Replace with file: ', or something, so that if a user uploads a new file, you can delete and replace the current file...