Gets the related files of the embedded file (RF, the array under the same key EmbeddedFile comes from).
public IReadOnlyList<PdfRelatedFile> RelatedFiles { get; }
ISO 32000-2 §7.11.3, Table 43 (PDF 1.3), and §7.11.4.2: an array of name strings, each followed by an embedded file stream.
A pair whose name is not a string or whose file is not a stream is skipped with a RelatedFilesInvalid diagnostic.