I've just made it the same manner as [storage configuration](https://github.com/mshibuya/carrierwave/blob/7d8301c26beb572a6a398788797afd2a8abb7801/lib/carrierwave/uploader/configuration.rb#L73), and the value of `storage_engines[storage]` can be either `"CarrierWave::Storage::File"` or `"CarrierWave::Storage::Fog"` by default.
Tracing back this line revealed that this change was introduced by d1c41567e7d38e3d38f00b1142862e4120d0ca32,  which states that the purpose is to prevent unnecessary autoloading of classes.