Attach object using iOS6 UIActivityViewController

I'm migrating to use the UIActivityViewController for sharing in iOS6, but I can't figure out how to create email attachment objects to be included when sharing by email.

The corresponding code in iOS5 is:

MFMailComposeViewController *picker = [[MFMailComposeViewController alloc] init];
[picker addAttachmentData:data mimeType:@"application/XXX" fileName:fileName];