All of supported web application frameworks(Merb, Rails, Sinatra, Padrino) has `/tmp` directory at the same level with `/public`, so we can safely assume that `../tmp` of CarrierWave.root(=`/public`) as default location of tmp directory.
If that's not the case or someone wants to use tmp directory other than the framework's default, he or she can specifically set tmp_path such as `CarrierWave.tmp_path = '/path/to/tmp'` in app's initializer or somewhere around.