What is the best place to store temporary files for WP plugin which are not necessary for the future? And what is the best place to store files which will be downloaded via FTP?
You could create your own file in wp-content and reference it like so:
$cache = content_url(); $cache .= "/cache/"
That should echo
http://mydomain.com/wp-content/cache/ so you can run all your operations off of that, like systematically clear it.
Do the same for downloads if you want them to be distinct.