Appendix A. Differences between HTTP and MIME
HTTP/1.1 uses many of the constructs defined for the Internet Message Format [RFC5322] and the Multipurpose Internet Mail Extensions (MIME) [RFC2045] to allow a message body to be transmitted in an open variety of representations and with extensible header fields. However, RFC 2045 is focused only on email; applications of HTTP have many characteristics that differ from email; hence, HTTP has features that differ from MIME. 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 MIME. Proxies and gateways to and from strict MIME environments need to be aware of these differences and provide the appropriate conversions where necessary.
A.1. MIME-Version
HTTP is not a MIME-compliant protocol. However, messages can include a single MIME-Version header field to indicate what version of the MIME protocol was used to construct the message. Use of the MIME-Version header field indicates that the message is in full conformance with the MIME protocol (as defined in [RFC2045]). Senders are responsible for ensuring full conformance (where possible) when exporting HTTP messages to strict MIME environments.
A.2. Conversion to Canonical Form
MIME requires that an Internet mail body part be converted to canonical form prior to being transferred, as described in Section 4 of [RFC2049]. Section 3.1.1.3 of this document describes the forms allowed for subtypes of the "text" media type when transmitted over HTTP. [RFC2046] requires that content with a type of "text" represent line breaks as CRLF and forbids the use of CR or LF outside of line break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a line break within text content.
A proxy or gateway from HTTP to a strict MIME environment ought to translate all line breaks within the text media types described in Section 3.1.1.3 of this document to the RFC 2049 canonical form of CRLF. Note, however, this might be complicated by the presence of a Content-Encoding and by the fact that HTTP allows the use of some charsets that do not use octets 13 and 10 to represent CR and LF, respectively.
Conversion will break any cryptographic checksums applied to the original content unless the original content is already in canonical form. Therefore, the canonical form is recommended for any content that uses such checksums in HTTP.
A.3. Conversion of Date Formats
HTTP/1.1 uses a restricted set of date formats (Section 7.1.1.1) to simplify the process of date comparison. Proxies and gateways from other protocols ought to ensure that any Date header field present in a message conforms to one of the HTTP/1.1 formats and rewrite the date if necessary.
A.4. Conversion of Content-Encoding
MIME does not include any concept equivalent to HTTP/1.1's Content-Encoding header field. Since this acts as a modifier on the media type, proxies and gateways from HTTP to MIME-compliant protocols ought to either change the value of the Content-Type header field or decode the representation before forwarding the message. (Some experimental applications of Content-Type for Internet mail have used a media-type parameter of ";conversions=<content-coding>" to perform a function equivalent to Content-Encoding. However, this parameter is not part of the MIME standards).
A.5. Conversion of Content-Transfer-Encoding
HTTP does not use the Content-Transfer-Encoding field of MIME. Proxies and gateways from MIME-compliant protocols to HTTP need to remove any Content-Transfer-Encoding prior to delivering the response message to an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols are responsible for ensuring that the message is in the correct format and encoding for safe transport on that protocol, where "safe transport" is defined by the limitations of the protocol being used. Such proxies or gateways ought to transform and label data with an appropriate Content-Transfer-Encoding if doing so will improve the likelihood of safe transport over the destination protocol.
A.6. MHTML and Line Length Limitations
HTTP implementations that share code with MHTML [RFC2557] implementations need to be aware of MIME line length limitations. Since HTTP does not have this limitation, HTTP does not fold long lines. MHTML messages being transported by HTTP follow all conventions of MHTML, including line length limitations and folding, canonicalization, etc., since HTTP transports all message-bodies as payload and, aside from the "multipart/byteranges" type (Appendix A of [RFC7233]), does not interpret the content or any MIME header lines that might be contained therein.