An invention that now seems to be ubiquitous turns out to only be 20 years old. RFC 1049 introduced the notion of a standardized Content-type field that message processors could use to automatically identify the type of a structured message and interpret it appropriately. This same convention went on to show up in the Multipurpose Internet Mail Extensions (MIME) message format, HTTP, and other later protocols. It commonly continues to appear with its original name, content type, although you'll sometimes hear other phrases used instead, such as MIME type or media type.

The original content type had a set of seven supported values.

1. Postscript, a document description language that was the most prevalent format for exchanging rendered documents prior to the introduction of PDF.
2. Scribe, a now obsolete word processing language that was marketed by the Unilogic corporation.
3. SGML, a language for describing other markup languages. SGML was used in the creation of HTML and XML, which quickly led to those languages becoming preferred over having to use SGML.
4. TeX, a typesetting language developed by Donald Knuth. Usage of plain TeX almost vanished as several rich TeX macro packages were developed, including LaTeX by Leslie Lamport.
5. Troff, a document processing system that would have died out if not for its use as the default format for writing UNIX manual pages.
6. DVI, a simple, device independent document layout format that was typically seen as an intermediate product when converting TeX or Troff files to Postscript.
7. X-, the convention for starting a user-defined content type, inherited from the convention for user-defined mail headers.