MIME is backwards compatable to RFC 822. Therefore a header field is required to define the type of data contained in the message body. The first three header fields below are required and the last two are optional.
MIME-Version - Describes the version of the MIME message format.
Content-Type - Describes the MIME content type and subtype. There are seven defined types which include:
Application - Executable or binary code is being transmitted which a specific program may be required to run.
Audio - Audio data.
Image - Graphic non-moving images.
Message - Encapsulates a message as defined by RFC 822.
Multipart - Combines several parts of the document which have different types.
Text - (Default) Specifies text content.
Video - Moving images.
A subsequent page lists the subtypes in more detail.
Content-Transfer-Encoding - Endicates the encoding method used to transform the document into seven bit format for transport. Options:
7-Bit - Data is sent as US-ASC data.
8-Bit - 8 bit characters are included in short lines.
BASE64 - Used for binary files. Three bytes are transformed into 4 ASC characters in lines limited to a length of 76 characters.
Binary - Long lines are sent using 8 bit characters. These lines may not be transportable using SMTP.
Quoted-Printable - Used tor ASC text. Line length is limited to 76 characters.
X-Token - Defines private encoding values prefixed with an "X-".
Content-ID - Allows a body of information to refer to another.
Content-Description - Possible description for a body of information.