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MIME Email Attachment Encoding

Multipurpose Internet Mail Extensions (MIME) has become the most widely supported attachment encoding standard for supporting multimedia formats.

Before the development of MIME, there had been a previous attempt to develop a multi-media capability for ARPANET email, but it was too complex and did not succeed. The MIME standard was a success because it added multi-media extensions to the existing email system without changing the infrastructure -- an important feature for any addition to an operational service with a large installed base.

The original impetus behind the development of MIME was the need to transmit "international" character sets in Internet mail, beyond the standard ASCII character set, to support international alphabets and symbols. Nathaniel Borenstein at Carnegie Mellon University led the design of MIME to handle international character sets, and an intentional side-effect of this design was the capability to extend the labeling scheme to different types of content, including multimedia content.

The modern MIME encoding standard was first defined in paragraph 4.3 of RFC 989, updated by paragraph 4.3.2.4 of RFC 1421, and now includes elements like Internet links inline to the email message body. MIME encodes a file into the following 64 alphanumeric characters:

ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz0123456789+/

If your email application gives you a choice of which encoding system to use for email attachments, the MIME standard is almost always the best selection. MIME allows a wide range of file types to be safely encoded and decoded, and is now recognized by almost all email applications.

Resources. Request For Comment's describing MIME are listed below:

  • RFC 1341; MIME: Mechanisms for Specifying and Describing the Format of Internet Message Bodies; N. Borenstein, N. Freed; June 1992. Updated by RFC 1521 and RFC 1590.
  • RFC 2045; Multipurpose Internet Mail Extensions Part One: Format of Internet Message Bodies; N. Freed, N. Borenstein; November 1996.
  • RFC 2046; Multipurpose Internet Mail Extensions Part Two: Media Types; N. Freed, N. Borenstein; November 1996.
  • RFC 2047; Multipurpose Internet Mail Extensions Part Three: Message Header Extensions for Non-ASCII Text; K. Moore; November 1996.
  • RFC 2048; Multipurpose Internet Mail Extensions Part Four: Registration Procedures; N. Freed, J. Klensin, J. Postel; November 1996.
  • RFC 2049; Multipurpose Internet Mail Extensions Part Five: Conformance Criteria and Examples; N. Freed, N. Borenstein; November 1996.
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