MIME stands for Multipurpose Internet Mail Extension and is an Internet standard that extends the format of email to support:

  • Text in character sets other than ASCII
  • Non-text attachments
  • Message bodies with multiple parts
  • Header information in non-ASCII character sets

MIME's use, however, has grown beyond describing the content of email and now is often used to describe content type in general including for the web and as a storage for rich content in some commercial products.

Reference http://codex.wordpress.org/Glossary