One or more attachments can be included in an e-mail using the MIME Adapter. Each attachment is defined as a single MIME part, which is the basic building block of the MIME format (MIME stands for Multipurpose Internet Mail Extensions).
Even the most complicated MIME message boils down to a collection of MIME parts, which can be arranged in a hierarchical structure. The MIME specification refers to messages that contain multiple documents as multipart parts. Once the multipart parts have been created, the SMTP header parameter like MIME version and content type are added before sending the e-mail. The content type is multipart/mixed, a multipart type typically used for e-mail attachments.
For more information on the MIME part structure, refer to the documenation of the MIME Adapter.
The activity diagram below shows, how attachments are passed to the SMTP server as MIME part input objects of different base types, e.g. String and Blob.
Figure: Sending E-mails with Attachments Activity Diagram