Formerly known as MIME (Multipurpose Internet Mail Extension) types. These identify different types of files allowing software to handle them correctly. Puppy uses the standard provided by freedesktop.org.

How To Add MIME Type
How to Associate Files to programs
Microsoft Office MIME-types
shared-mime-info - MIME-Type descriptions from freedesktop.org
MIME Types and File Extensions
WikiPedia:Internet_media_type
Mime icons for ROX-Filer thread