To save NSAttributedString string into text file we convert first NSAttributedString into Data and afterwards save Data as usual into a text file:
do { let rtfData = try attributedText.data(from: .init(location: 0, length: attributedText.length), documentAttributes: [.documentType: NSAttributedString.DocumentType.rtf]) try rtfData.write(to: fileURL, options: .atomic) } catch { print(error) }