Multipurpose Internet Mail Extensions (MIME) is a mechnism to extend email beyond text-based content.

Initially defined in RFC 2045, it allows for:

  • Other text encodings beyond US-ASCII
  • Multi-part message bodies
  • An extensible set of different formats for non-textual message bodies

Individual message parts are described with a so called “media type” (also known as “MIME type”) as specified in related RFC 2046. Media types are strings, consisting of the general type of data in combination with a more specific “subtype” (e.g., text/html, image/jpeg).

Nowadays, media types are ubiquitous also in web browsers and operating systems. complementing file name extensions in determining the type and purpose of files.

With respect to email, media types are used in various different ways:

  • Text and image media types play a crucial role in a more “rich” email experience (HTML email) and are directly displayed to the user by the email client software
  • A number of media types have been specially designed for certain email-specific workflows (see also RFCs)
  • The majority of media types is not directly supported by the email client, but defered to external applications

Accordingly, MIME can be considered an extension system of email, which allows to pass files to other applications. This application is also colloquially referred to as “email attachments”.