A file upload field holds the ID of the file which is saved as an attachment post type. There isn’t any functionality in this field to allow for what you want to do. You would need to use another type of field for a link. In this case I would create a radio field to allow the editor to select “upload file” or “external link to file” and then I would create a file field and a url field that both had conditional logic to display based on the selection of the radio field.
