Messages so far, more exactly RFC822 messages, cannot contain objects other than text. MIME is multi-purpose message to extend RFC822.
MIME has
MIME-Version: 1.0
field in its header. Without this field, it is an RFC822 message. In MIME, Content-Type: to indicate data type and Content-Transfer-Encoding to specify encoding are important fields. The following sections describe these fields and feature of MIME.