In addition to the XML payload, a SOAP message can carry binary data, such as an image or an audio file in the form of an attachment. The binary data is packaged with the SOAP message. After packaging, the data is sent as a single stream over any of the transport protocols, such as HTTP, SMTP, or the File Transfer Protocol, or FTP.

The SOAP Messages with Attachments, or SwA, specification describes how to attach binary data to a SOAP message. Defined by the World Wide Web Consortium, or W3C, in the SOAP 1.2 Attachment Features document, SwA is an abstract model for packaging attachments with a SOAP message. SwA is designed as a compound document structure that consists of a primary SOAP message part and zero or more secondary attachment parts.

SwA is primarily implemented through Multipurpose Internet Mail Extension, or MIME, and Direct Internet Message Encapsulation, or DIME. While MIME is a protocol that defines the standard for sending non-text files as email messages, DIME is an improvement over MIME.