Gets or sets when the file was last modified (Params ModDate); null writes none.
public DateTimeOffset? ModificationDate { get; set; }
ISO 32000-2 §7.11.4.1, Table 45, and §7.9.4. §14.13.2: an associated file's parameters should carry it, so set it for a file that will be attached with AddAssociatedFile(CosDictionary, PdfFileSpecification).