Support ms-tnef attachments for embedded OL items

* src/attachment.cpp (Attachment::Attachment): Fall back to
save_as_file when attachment is not available in oom.
* src/mimemaker.cpp (infer_content_type): Infer ms-tnef for .msg
attachments.

This allows attaching mails, contacts and events to encrypted /
signed messages using the new attachment handling code that
works in memory and outside of MAPI.

• GnuPG-Bug-Id: T4184