Transact XML supports attachments. Transact SMTP does not.
Specify the disposition of the transact attachment in the XML request. Inline means that the file is embedded in the body of the email. Attachment means that the file is sent with the email. If the disposition is not specified, the default is Attachment. The positioning of an inline file cannot be specified.
For more information, see Getting Started with Transact Attachments in the Developer area.