MIME

Original email could carry only plain English text — strictly 7-bit ASCII letters. No images, no PDFs, no Chinese characters, no song attached to a message. MIME is the clever wrapper that lets all of that travel through a system built only for text. It works by encoding any kind of binary data into safe text characters and labelling what it really is, so a photo can ride inside a plain-text email and be reassembled into a photo at the other end.

MIME does two jobs. First, it labels content with a media type (also called a MIME type), written as type/subtype: text/plain, text/html, image/jpeg, application/pdf, application/json. This label tells the receiver how to interpret the bytes — show it as text, render it as a webpage, open it as a PDF. Second, for data that is not plain text, MIME encodes it into ASCII, most often with Base64, which represents arbitrary bytes using only safe letters, digits, and a couple of symbols. A single email can also be multipart — a multipart/mixed message bundles, say, a text/plain note plus an image/jpeg attachment, each part with its own type header, separated by a boundary marker.

MIME's reach goes far beyond email. The exact same media-type system labels content on the web: the HTTP "Content-Type: text/html" or "Content-Type: image/png" header is a MIME type, telling your browser whether the bytes are a page, a picture, or a download. So MIME quietly underpins both how an attachment survives an email system and how a browser knows what it just received. The honest cost is size: Base64 encoding inflates binary data by about a third, since it spends extra characters to stay text-safe — a deliberate trade of efficiency for compatibility with old text-only infrastructure.