Type: String
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)
Added in protocol FIX.5.0SP2 (EP 167)