For its first two decades, email could carry only words. It was pure seven-bit ASCII text, and that was the whole of what a message could hold: no images, no sound, no attachments of any kind. A letter was letters, and nothing else fit through the pipe. The restriction seems almost unimaginable now, but it was simply the shape of the medium, a channel built to move plain characters and unbothered about anything richer.

The thing that opened the envelope was a specification. MIME, published as RFC 1341 in 1992 by Nathaniel Borenstein and Ned Freed, taught messages to declare what they carry. Instead of assuming every message was plain text, MIME let a message announce its own contents, labelling each part so the receiving software would know whether it held a picture, a sound, a document, or ordinary prose. That simple idea of self-declaration is what let email grow past words and become the carrier of everything we now send.

What is easy to miss is how far those labels travelled. The vocabulary MIME invented did not stay inside email. Its labels became this entire registry, the shared list of types that names every kind of file the network moves. And it reached further still. Every Content-Type header on the web, the small line of text that tells a browser whether it is receiving a page, an image, or a video, descends directly from that 1992 specification. Each one is, in effect, an airmail sticker that escaped the post office: a label born to say what a letter contained, now affixed to nearly everything that crosses the internet. The exhibit takes its name and its instinct from that moment, when a message first learned to introduce itself, and the small act of declaration turned out to be the thing on which the whole rich web would come to depend.