OptionalContentContentType.ExtractorKey (or ContentSourceType.Configuration.ExtractorKey).
OptionalExtractorContentItem.ExtractorKeyOverride, or one a splitting extractor stamped on its child.
The resolved file type. An extractor that does not support it is skipped.
OptionalFileContentFileType.ExtractorKey.
OptionalRunThe run's Configuration.ExtractorKey.
OptionalSourceThe source's Configuration.Extractors[FileType] or Configuration.ExtractorKey.
The cascade's rungs, most specific first.