Learn in the textbookHow Email Works and Specifying Recipients
DefinitionMIME is an extension standard that lets email, once limited to text, carry many kinds of data such as images and audio.
ExplanationData such as an attached file is first encoded as a string of characters, then sent together with the body as one message. Separately, the common body formats are plain text (テキスト形式), which holds characters only, and HTML format (HTML 形式), which allows formatting, and either one can carry attachments.
AnalogyImagine a postal service that carries only envelopes: you fold a piece of cloth to fit inside one, and the receiver unfolds it at the other end.
Memory hookM is Multipurpose and E is Extensions: the name itself says text-only email was widened to many uses.