Lecture 4: Applications #2: Email

MIME Types and Encodings

The Content-Type: header in MIME specifies a "MIME type" for the data which follows. The MIME type is used to open a suitable application program to display the attached data. Some standard MIME types include:

`text/plain`	lines of ASCII text
`text/html`	HTML text
`image/gif`	GIF image
`video/mpeg`	MPEG video
`application/postscript`	PostScript document
`application/octet-stream`	Arbitrary data

For non-ASCII (8 bit) data, common encodings include "quoted-printable" and "Base64".

In Base64 encoding, the message is subdivided into groups of 3 bytes (24 bits) in length. These 24 bits are then subdivided into 4 groups of 6 bits each. Each 6 bit group is represented as one of 64 printable ASCII characters, from the 95 printable characters in ASCII. Finally, each of the printable characters is sent as an 8 bit byte. Thus, 24 bits of data are sent as 32 bits of ASCII data in the encoded message.

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