basically every file is a download. So if you go to google.com the HTML and Pictures are a download, exactly like downloading an ISO image. All these requests go into the access.log. You will need a criteria to decide which files should go into your "extra download".log. I would recommend something like all files bigger than 2 MB. You can create a rule that says "If Body.Size greater than 2MB then Continue" and configure "File System Logging" as an event to write a new line into a different log.