Always using MIME-Type 'application/octet-stream' is not optimal. Most if not all browsers will simply download files with that type.
If you use proper MIME types (and inline Content-Disposition), browsers will have better default actions for some of them. Eg. in case of images, browsers will display them, which is probably what you'd want.
To deliver the file with the proper MIME type, the easiest way is to use:
header('Content-Type: ' . mime_content_type($file));
header('Content-Disposition: inline; filename="'.basename($file).'"');