MIME, the Multipurpose Internet Mail Extensions, is the standard that gave email the ability to carry more than flat seven-bit ASCII text. It was introduced in RFC 1341, “MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies,” written by Nathaniel Borenstein of Bellcore and Ned Freed of Innosoft and published in June 1992. The document set out to fix concrete limitations of the older RFC 822 message format, which left the message body as flat ASCII text and could not properly represent images, audio, video, binary files, or the richer character sets needed by languages beyond US English.
MIME solves this without changing the underlying mail transport at all. SMTP still moves the same kind of message envelope it always did; MIME works entirely at the message-format layer by adding a small set of header fields that describe and structure the content. The key fields introduced by RFC 1341 are MIME-Version, which marks a message as conforming to the standard; Content-Type, which labels each body part with a type and subtype such as text/plain, image/jpeg, or application/octet-stream; and Content-Transfer-Encoding, which specifies how binary or eight-bit data has been encoded into a form that survives transport over channels that assume seven-bit text.
The most visible MIME innovation is the multipart message. A Content-Type of multipart lets a single message contain several distinct body parts separated by a boundary marker, so an email can hold a text note alongside one or more attachments, or offer the same content in alternative forms such as plain text and formatted text. This multipart structure is what users experience as email attachments, and it is the mechanism behind nearly every file ever mailed.
To carry binary data safely through systems built for ASCII, MIME standardized encodings such as base64, which represents arbitrary bytes using a limited alphabet of printable characters, and quoted-printable, which keeps mostly-text content readable while escaping the occasional non-ASCII byte. These encodings let a recipient reconstruct the exact original bytes even after the message has passed through gateways that would otherwise mangle them.
MIME was refined and split into a family of documents, with RFC 2045 and its companions later replacing RFC 1341 as the core specifications, but the design was stable enough that its central concept spread far beyond email. The Content-Type header and the registry of MIME types it relies on were adopted by HTTP to label the documents and media that web servers send to browsers. As a result, every time a browser correctly displays an image, a stylesheet, or a video, it is relying on the same media-type vocabulary that Borenstein and Freed created to fix email in 1992.