Associates file with owner by adding it to the owner's AF array.
public void AddAssociatedFile(CosDictionary owner, PdfFileSpecification file)
The dictionary the file is associated with: the Catalog, a page's Dictionary, an annotation, a structure element, an XObject's stream dictionary, a document part or a marked-content property list.
A file specification dictionary of this document; its AFRelationship says how the file relates to the owner.
ISO 32000-2 §14.13.1 to §14.13.9 (PDF 2.0): AF is an array of file specification dictionaries; the specification is
added by its indirect reference when it has one. A file already in the array is not added twice. An AF that is a single
dictionary (read leniently as an array of one) becomes an array holding it and the new file; an AF that is neither is
replaced. An embedded associated file should have a Subtype and Params with ModDate (§14.13.2), which
CreateEmbeddedFile(String, ReadOnlySpan<Byte>, PdfEmbeddedFileOptions?) writes when ModificationDate is set. Minimum version PDF
2.0, which a later save raises the document to (or refuses in strict mode). A mutation: no read may run concurrently with it.