array $params - An associative array of parameters:
one of:
$params['file'] - filepath
$params['data'] - raw data
$params['resource'] - resource handle
and any of:
$params['gzip'] - whether to gzip the download
$params['cache'] - whether to allow client side caching of the download
$params['lastmodified'] - unix timestamp of last modification
$params['contenttype'] - content type
$params['contentdisposition'] - content disposition
$params['buffersize'] - amount of bytes read at once from files or resources
$params['throttledelay'] - amount of seconds to sleep after each chunk that has been sent
bool [$guess = false] - whether to call guessContentType()