MIME means Multipurpose Internet Mail Extensions, and refers to an official Internet standard that specifies how messages must be formatted so that they can be exchanged between different email systems. MIME is a very flexible format, permitting one to include virtually any type of file or document in an email message. Specifically, MIME messages can contain text, images, audio, video, or other application-specific data.
To insure that email messages containing images or other non-text information will be delivered with maximum protection against corruption, MIME provides a way for non-text information to be encoded as text. This encoding is known as base64, and appears to be a source of frustration for many email users.
The definitive information on the MIME format is provided by the Internet Engineering Task Force (IETF) in the following documents: