188.8.131.52.2 Apple File Formats
[RFC1740] and [RFC1741] specify the use of the MIMEContent-Types "multipart/appledouble", "application/applefile", and "application/mac-binhex40" to encode files that originate from a Macintosh operating system, to preserve additional data that might be available for these files in that operating system. MIME readers SHOULD preserve this additional data for attached files to enable full support of Macintosh-based client applications.
In particular, the Attachment object content that is stored in the PidTagAttachDataBinary property ([MS-OXCMSG] section 184.108.40.206) MUST contain a MacBinary stream (1). This stream (1) format incorporates both the resource and data forks, as well as certain metadata.