How do you copy an NSAttributedString in the pasteboard, to allow the user to paste, or to paste programmatically (with - (void)paste:(id)sender
The pasteboard manager in OSX can auto convert between a lot of textual and image types.
For rich text types, you'd usually place RTF into the pasteboard. You can create RTF representation from an attributed string, and vice versa. See the "NSAttributedString Application Kit Additions Reference".
If you have images included as well, then use the RTFd instead of RTF flavor.
I don't know the MIME types for these (I'm used to the Carbon Pasteboard API, not the Cocoa one), but you can convert between UTIs, Pboard and MIME Types using the UTType API.
UTI for RTF is "public.rtf", for RTFd it's "com.apple.flat-rtfd".