The MIME media type (and optional subtype) of the attachment. The values used are those assigned, listed and maintained by IANA (www.iana.org) [RFC2046]. See http://www.iana.org/assignments/media-types/index.html for available types.
Examples values (RFC number provided for reference here only):
application/pdf (see [RFC3778])
application/msword (for .doc files)
multipart/signed (see [RFC1847])
application/vnd.openxmlformats-officedocument.wordprocessingml.document (for .docx files)