Content Body Parts

Each content body part is made up of header fields and content. The headers convey specific information about the content for the message recipient. The content can be essentially any serialized stream of bytes, from a binary file to text formatted in Hypertext Markup Language (HTML). When necessary, the content is encoded so that the entire body complies with RFC 822. If the content is encoded, MIME defines the header Content-Transfer-Encoding to specify the mechanism. Other details are sometimes included, such as the Content-Disposition, which indicates to the recipient whether the content is to be treated simply as an attachment, or whether it is to be rendered inline with other content in other body parts.

For example, say you want to include a picture, which is stored as a file in Graphics Interchange Format (GIF) format, in a message. Because .gif files use 8-bit bytes, and the RFC 822 format requires messages to contain only US-ASCII text, the picture data must be encoded. For the recipient to correctly decode and display the picture, it needs information about which encoding mechanism was used. The following example shows part of a MIME header that identifies that the content is a .gif file, that it is encoded using the standard base64 algorithm, and that it is to be treated by the e-mail client as an attachment.

Content-Type: image/gif;
     name="picture.gif"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
     filename="picture.gif"
  
[encoded content here]