Optional ReadonlyblobThe received data as a Blob, when the sink materialised one.
Optional ReadonlybytesThe received data as raw bytes, when the sink materialised them.
Optional ReadonlypathFilesystem path the data was written to, for file-backed sinks.
Outcome of a completed transfer, returned by StorageSink.close.
Which fields are populated depends on the sink: in-memory sinks return
blob/bytes, filesystem sinks return apath.