Outside the Document: MIME and File Extensions

Identifiers without Instructions

MIME Originally developed for email, MIME content-types provide a two-part identification string for particular document types. Pluses: Widely implemented, a core part of HTTP processing. Minuses: Content-types are defined in human-language documents; application developers must handle processing themselves.
File Extensions The way most PC users identify their files, and a well-understood convention. Pluses: Everybody's doing it. Minuses: No central registry identifying what they mean.
XML Documents and External Identifiers While there was a brief bit of pleading that people not use '.xml' on files, no strong consensus has emerged.