Relationship between HTTP and MIME

The relationship between HTTP and MIME is defined in the HTTP/1.1 rfc 2616 section 19.4.

HTTP/1.1 uses many of the constructs defined for Internet
Mail (RFC 822 [9]) and the Multipurpose Internet Mail
Extensions (MIME [7]) to allow entities to be transmitted
in an open variety of representations and with extensible
mechanisms. However, RFC 2045 discusses mail, and HTTP has
a few features that are different from those described in
RFC 2045. These differences were carefully chosen to
optimize performance over binary connections, to allow
greater freedom in the use of new media types, to make
date comparisons easier, and to acknowledge the practice
of some early HTTP servers and clients.

This appendix describes specific areas where HTTP differs
from RFC 2045. Proxies and gateways to strict MIME
environments SHOULD be aware of these differences and
provide the appropriate conversions where necessary.
Proxies and gateways from MIME environments to HTTP also
need to be aware of the differences because some
conversions might be required.