2.1.3.3.11 attAttachData Attribute
The attAttachData attribute is an attachment-level attribute that maps to and from the PidTagAttachDataBinary property ([MS-OXCMSG] section 2.2.2.7).
The data for this attribute is stored as a binary stream.
For details about how the TNEF Writer handles this attribute, see section 2.2.3.8. For details about how the TNEF Reader handles this attribute, see section 2.3.3.7.