[eluser]whitey5759[/eluser]
I might be wrong but I don't think that is the purpose of those 'cache' folders. Aren't they used for caching by the Output and Database libraries, built into CI?
The way I have done it is to create a directory in the sites root (same level as index.php, system, etc) called say 'downloads' or whatever, and then add this directory name to your 'RewriteCond' statement in your .htaccess file, eg: RewriteCond $1 !^(index\.php|assets|downloads|robots\.txt)