Pattern where processing runs when the file is uploaded/attached, producing a collection of derivative files that are each persisted (upload location serialized per file). Common for larger artifacts (e.g. videos) that can't feasibly be processed on the fly. In Shrine, the processing block is generic Ruby and can perform arbitrary transformations (e.g. video transcoding) without needing plug-ins that know library internals. Persisting each derivative's upload location means changing the path scheme later doesn't invalidate existing URLs.