Returns the folder an embedded file belongs to: the folder its key's <id> prefix names, else the root folder.
public PdfCollectionFolder? GetFolder(PdfEmbeddedFileEntry file)
An entry of EmbeddedFiles.
null when the collection has no folders.ISO 32000-2 §12.3.5, "Folders": a key without a valid folder prefix, or naming no folder, places the file in the root.