How can I save the attributed string (text) into rtf file?

I have NSTextView and I can have text as nsattributedstring. I can save text into .txt file using NSSavePanel, as plain text, but not as formatted text.

[code] @IBAction func saveDNA(sender: AnyObject)
let saveDNAtoFile: NSSavePanel = NSSavePanel()
saveDNAtoFile.canSelectHiddenExtension = true

    let exportedFileURL = saveDNAtoFile.URL
    let textDNA = self.inputDnaFromUser.string

    if exportedFileURL != nil
        try textDNA!.writeToURL(exportedFileURL!, atomically: false, encoding: NSUTF8StringEncoding)
} catch

How can I save the attributedstring (text) into file using NSSavePanel, to be able later to open this file to have all made before formatting in the text? What I should change in the code above, if I can use NSSavePanel for this ?