let attachments = NSMutableDictionary(dictionary: CMCopyDictionaryOfAttachments(nil, buffer, kCMAttachmentMode_ShouldPropagate) as! NSDictionary)

let exif = NSMutableDictionary(dictionary: attachments[kCGImagePropertyExifDictionary] as! NSDictionary)

exif[kCGImagePropertyExifMakerNote] = "Blah blah"

attachments[kCGImagePropertyExifDictionary] = exif

exiftool photo.JPG | grep -i maker

exiftool photo.JPG | grep -i blah

let comment = ["My custom setting": "blah"]

exif[kCGImagePropertyExifUserComment] = comment