MIME

Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format of e-mail 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 e-mail to describing content type in general, including for the web (see Internet media type).
Virtually all human-written Internet e-mail and a fairly large proportion of automated e-mail is transmitted via SMTP in MIME format. Internet e-mail is so closely associated with the SMTP and MIME standards that it is sometimes called SMTP/MIME e-mail.[1]
The content types defined by MIME standards are also of importance outside of e-mail, such as in communication protocols like HTTP for the World Wide Web. HTTP requires that data be transmitted in the context of e-mail-like messages, although the data most often is not actually e-mail.
MIME is specified in six linked RFC memoranda: RFC 2045, RFC 2046, RFC 2047, RFC 4288, RFC 4289 and RFC 2049, which together define the specifications.