MultipurposeInternetMailExtensions, more commonly known as MIME, is a simple yet generic way of representing multiple data sources in one e-mail (RFC2822/RFC822) compatible stream.

1. RFC2045 - Format of Internet Message Bodies
2. RFC2046 - Media Types
3. RFC2047 - Message Header Extensions for Non-ASCII Text
4. RFC2048 - Registration Procedures
5. RFC2049 - Conformance Criteria and Examples

You may also be interested in:

1. RFC1524 - The formal description of mailcap files. Mailcap files describe how to handle media types
2. RFC2015 - MIME Security with Pretty Good Privacy (PGP)
3. RFC2110 - MIME E-mail Encapsulation of Aggregate Documents, such as HTML (MHTML)
4. RFC2111 - Content-ID and Message-ID Uniform Resource Locators
5. RFC2112 - The MIME Multipart/Related Content-type
6. RFC2183 - Defines the syntax and sematics of the "Content-Disposition" header to convey presentational information
7. RFC2184 - MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations

More recently, MIME has been XML-ized/HTTP-ized as BlockExtensibleExchangeProtocol (BEEP/BXXP). This is not to say that BXXP is MIME, but it's clear that BXXP was influenced by HTTP which was influenced by MIME.