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Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format of e-mail to support:
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.
Contents |
The basic Internet e-mail transmission protocol, SMTP, supports only 7-bit ASCII characters (see also 8BITMIME). This effectively limits Internet e-mail to messages which, when transmitted, include only the characters sufficient for writing a small number of languages, primarily English. Other languages based on the Latin alphabet typically include diacritics not supported in 7-bit ASCII, meaning text in these languages cannot be correctly represented in basic e-mail.
MIME defines mechanisms for sending other kinds of information in e-mail. These include text in languages other than English using character encodings other than ASCII, and 8-bit binary content such as files containing images, sounds, movies, and computer programs. MIME is also a fundamental component of communication protocols such as HTTP, which requires that data be transmitted in the context of e-mail-like messages even though the data might not (and usually doesn't) actually have anything to do with e-mail. Mapping messages into and out of MIME format is typically done automatically by an e-mail client or by mail servers when sending or receiving Internet (SMTP/MIME) e-mail.
The basic format of Internet e-mail is defined in RFC 5322, which is an updated version of RFC 2822 and RFC 822. These standards specify the familiar formats for text e-mail headers and body and rules pertaining to commonly used header fields such as "To:", "Subject:", "From:", and "Date:". MIME defines a collection of e-mail headers for specifying additional attributes of a message including content type, and defines a set of transfer encodings which can be used to represent 8-bit binary data using characters from the 7-bit ASCII character set. MIME also specifies rules for encoding non-ASCII characters in e-mail message headers, such as "Subject:", allowing these header fields to contain non-English characters.
MIME is extensible. Its definition includes a method to register new content types and other MIME attribute values.
The goals of the MIME definition included requiring no changes to existent e-mail servers and allowing plain text e-mail to function in both directions with existing clients. These goals were achieved by using additional RFC 822-style headers for all MIME message attributes and by making the MIME headers optional with default values ensuring a non-MIME message is interpreted correctly by a MIME-capable client. A simple MIME text message is therefore likely to be interpreted correctly by a non-MIME client although if it has e-mail headers the non-MIME client won't know how to interpret. Similarly, if the quoted printable transfer encoding (see below) is used, the ASCII part of the message will be intelligible to users with non-MIME clients.
The presence of this header indicates the message is MIME-formatted. The value is typically "1.0" so this header appears as
MIME-Version: 1.0
It should be noted that implementers have attempted to change the version number in the past and the change had unforeseen results. It was decided at an IETF meeting[2] to leave the version number as is even though there have been many updates and versions of MIME.
The Content-ID header is primarily of use in multi-part messages (as discussed below); a Content-ID is a unique identifier for a message part, allowing it to be referred to (e.g., in IMG tags of an HTML message allowing the inline display of attached images). The content ID is contained within angle brackets in the Content-ID header. Here is an example:
Content-ID: <5.31.32252.1057009685@server01.example.net>
The standards don't really have a lot to say about exactly what is in a Content-ID; they're only supposed to be globally and permanently unique (meaning that no two are the same, even when generated by different people in different times and places). To achieve this, some conventions have been adopted; one of them is to include an at sign (@), with the hostname of the computer which created the content ID to the right of it. This ensures the content ID is different from any created by other computers (well, at least it is when the originating computer has a unique Internet hostname; if, as sometimes happens, an anonymous machine inserts something generic like localhost, uniqueness is no longer guaranteed). Then, the part to the left of the at sign is designed to be unique within that machine; a good way to do this is to append several constantly-changing strings that programs have access to. In this case, four different numbers were inserted, with dots between them: the rightmost one is a timestamp of the number of seconds since January 1, 1970, known as the Unix epoch; to the left of it is the process ID of the program that generated the message (on servers running Unix or Linux, each process has a number which is unique among the processes in progress at any moment, though they do repeat over time); to the left of that is a count of the number of messages generated so far by the current process; and the leftmost number is the number of parts in the current message that have been generated so far. Put together, these guarantee that the content ID will never repeat; even if multiple messages are generated within the same second, they either have different process IDs or a different count of messages generated by the same process.
That's just an example of how a unique content ID can be generated; different programs do it differently. It's only necessary that they remain unique, a requirement that is necessary to ensure that, even if a bunch of different messages are joined together as part of a bigger multi-part message (as happens when a message is forwarded as an attachment, or assembled into a MIME-format digest), you won't have two parts with the same content ID, which would be likely to confuse mail programs greatly.
There's a similar header called Message-ID which assigns a unique identifier to the message as a whole; this is not actually part of the MIME standards, since it can be used on non-MIME as well as MIME messages. If the originating mail program doesn't add a message ID, a server handling the message later on probably will, since a number of programs (both clients and servers) want every message to have one to keep track of them. Some headers discussed in the Other Headers article make use of message IDs.
When referenced in the form of a Web URI, content IDs and message IDs are placed within the URI schemes cid and mid respectively, without the angle brackets:
cid:5.31.32252.1057009685@server01.example.net
This header indicates the Internet media type of the message content, consisting of a type and subtype, for example
Content-Type: text/plain
Through the use of the multipart type, MIME allows messages to have parts arranged in a tree structure where the leaf nodes are any non-multipart content type and the non-leaf nodes are any of a variety of multipart types. This mechanism supports:
The original MIME specifications only described the structure of mail messages. They did not address the issue of presentation styles. The content-disposition header field was added in RFC 2183 to specify the presentation style. A MIME part can have:
In addition to the presentation style, the content-disposition header also provides fields for specifying the name of the file, the creation date and modification date, which can be used by the reader's mail user agent to store the attachment.
The following example is taken from RFC 2183, where the header is defined
Content-Disposition: attachment; filename=genome.jpeg;
modification-date="Wed, 12 Feb 1997 16:29:51 -0500";
The filename may be encoded as defined by RFC 2231.
As of 2010, a good majority of mail user agents do not follow this prescription fully. The widely used Mozilla Thunderbird mail client makes its own decisions about which MIME parts should be automatically displayed, ignoring the content-disposition headers in the messages. It also sends out newly composed messages with inline content-disposition for all MIME parts. Most users are unaware of how to set the content-disposition to attachment.[4] Many mail user agents also send messages where they put the file name in the name parameter of the content-type header instead of the filename parameter of the content-disposition header. This practice is discouraged.[5]
In June 1992, MIME (RFC 1341, since made obsolete by RFC 2045) defined a set of methods for representing binary data in ASCII text format. The content-transfer-encoding: MIME header has 2-sided significance:
The RFC and the IANA's list of transfer encodings define the values shown below, which are not case sensitive. Note that '7bit', '8bit', and 'binary' mean that no binary-to-text encoding on top of the original encoding was used. In these cases, the header is actually redundant for the e-mail client to decode the message body, but it may still be useful as an indicator of what type of object is being sent. Values 'quoted-printable' and 'base64' tell the e-mail client that a binary-to-text encoding scheme was used and that appropriate initial decoding is necessary before the message can be read with its original encoding (e.g. UTF-8).
There is no encoding defined which is explicitly designed for sending arbitrary binary data through SMTP transports with the 8BITMIME extension. Thus base64 or quoted-printable (with their associated inefficiency) must sometimes still be used. This restriction does not apply to other uses of MIME such as Web Services with MIME attachments or MTOM
Since RFC 2822, message header names and values are always ASCII characters; values that contain non-ASCII data must use the MIME encoded-word syntax (RFC 2047) instead of a literal string. This syntax uses a string of ASCII characters indicating both the original character encoding (the "charset") and the content-transfer-encoding used to map the bytes of the charset into ASCII characters.
The form is: "=?charset?encoding?encoded text?=".
Q" denoting Q-encoding that is similar to the quoted-printable encoding, or "B" denoting base64 encoding.The ASCII codes for the question mark (?) and equals sign may not be represented directly as they are used to delimit the encoded-word. The ASCII code for space may not be represented directly because it could cause older parsers to split up the encoded word undesirably. To make the encoding smaller and easier to read the underscore is used to represent the ASCII code for space creating the side effect that underscore cannot be represented directly. Use of encoded words in certain parts of headers imposes further restrictions on which characters may be represented directly.
For example,
Subject: =?iso-8859-1?Q?=A1Hola,_se=F1or!?=
is interpreted as "Subject: ¡Hola, señor!".
The encoded-word format is not used for the names of the headers (for example Subject). These header names are always in English in the raw message. When viewing a message with a non-English e-mail client, the header names are usually translated by the client.
A MIME multipart message contains a boundary in the "Content-Type: " header; this boundary, which must not occur in any of the parts, is placed between the parts, and at the beginning and end of the body of the message, as follows:
MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="frontier" This is a message with multiple parts in MIME format. --frontier Content-Type: text/plain This is the body of the message. --frontier Content-Type: application/octet-stream Content-Transfer-Encoding: base64 PGh0bWw+CiAgPGhlYWQ+CiAgPC9oZWFkPgogIDxib2R5PgogICAgPHA+VGhpcyBpcyB0aGUg Ym9keSBvZiB0aGUgbWVzc2FnZS48L3A+CiAgPC9ib2R5Pgo8L2h0bWw+Cg== --frontier--
Each part consists of its own content header (zero or more Content- header fields) and a body. Multipart content can be nested. The content-transfer-encoding of a multipart type must always be "7bit", "8bit" or "binary" to avoid the complications that would be posed by multiple levels of decoding. The multipart block as a whole does not have a charset; non-ASCII characters in the part headers are handled by the Encoded-Word system, and the part bodies can have charsets specified if appropriate for their content-type.
Notes:
The MIME standard defines various multipart-message subtypes, which specify the nature of the message parts and their relationship to one another. The subtype is specified in the "Content-Type" header of the overall message. For example, a multipart MIME message using the digest subtype would have its Content-Type set as "multipart/digest".
The RFC initially defined 4 subtypes: mixed, digest, alternative and parallel. A minimally compliant application must support mixed and digest; other subtypes are optional. Applications must treat unrecognised subtypes as "multipart/mixed". Additional subtypes, such as signed and form-data, have since been separately defined in other RFCs.
The following is a list of the most commonly used subtypes; it is not intended to be a comprehensive list.
Multipart/mixed is used for sending files with different "Content-Type" headers inline (or as attachments). If sending pictures or other easily readable files, most mail clients will display them inline (unless otherwise specified with the "Content-disposition" header). Otherwise it will offer them as attachments. The default content-type for each part is "text/plain".
Defined in RFC 2046, Section 5.1.3
A message/rfc822 part contains an e-mail message, including any headers. Rfc822 is a misnomer, since the message may be a full MIME message. This is used for digests as well as for E-mail forwarding.
Defined in RFC 2046.
Multipart/digest is a simple way to send multiple text messages. The default content-type for each part is "message/rfc822".
Defined in RFC 2046, Section 5.1.5
The multipart/alternative subtype indicates that each part is an "alternative" version of the same (or similar) content, each in a different format denoted by its "Content-Type" header. The formats are ordered by how faithful they are to the original, with the least faithful first and the most faithful last. Systems can then choose the "best" representation they are capable of processing; in general, this will be the last part that the system can understand, although other factors may affect this.
Since a client is unlikely to want to send a version that is less faithful than the plain text version, this structure places the plain text version (if present) first. This makes life easier for users of clients that do not understand multipart messages.
Most commonly, multipart/alternative is used for e-mail with two parts, one plain text (text/plain) and one HTML (text/html). The plain text part provides backwards compatibility while the HTML part allows use of formatting and hyperlinks. Most e-mail clients offer a user option to prefer plain text over HTML; this is an example of how local factors may affect how an application chooses which "best" part of the message to display.
While it is intended that each part of the message represent the same content, the standard does not require this to be enforced in any way. At one time, anti-spam filters would only examine the text/plain part of a message, because it is easier to parse than the text/html part. But spammers eventually took advantage of this, creating messages with an innocuous-looking text/plain part and advertising in the text/html part. Anti-spam software eventually caught up on this trick, penalizing messages with very different text in a multipart/alternative message.
Defined in RFC 2046, Section 5.1.4
A multipart/related is used to indicate that message parts should not be considered individually but rather as parts of an aggregate whole. The message consists of a root part (by default, the first) which reference other parts inline, which may in turn reference other parts. Message parts are commonly referenced by the "Content-ID" part header. The syntax of a reference is unspecified and is instead dictated by the encoding or protocol used in the part.
One common usage of this subtype is to send a web page complete with images in a single message. The root part would contain the HTML document, and use image tags to reference images stored in the latter parts.
Defined in RFC 2387
Multipart/report is a message type that contains data formatted for a mail server to read. It is split between a text/plain (or some other content/type easily readable) and a message/delivery-status, which contains the data formatted for the mail server to read.
Defined in RFC 3462
A multipart/signed message is used to attach a digital signature to a message. It has two parts, a body part and a signature part. The whole of the body part, including mime headers, is used to create the signature part. Many signature types are possible, like application/pgp-signature (RFC 3156) and application/x-pkcs7-signature (S/MIME).
Defined in RFC 1847, Section 2.1
A multipart/encrypted message has two parts. The first part has control information that is needed to decrypt the application/octet-stream second part. Similar to signed messages, there are different implementations which are identified by their separate content types for the control part. The most common types are "application/pgp-encrypted" (RFC 3156) and "application/pkcs7-mime" (S/MIME).
Defined in RFC 1847, Section 2.2
As its name implies, multipart/form-data is used to express values submitted through a form. Originally defined as part of HTML 4.0, it is most commonly used for submitting files via HTTP.
Defined in RFC 2388
The content type multipart/x-mixed-replace was developed as part of a technology to emulate server push and streaming over HTTP.
All parts of a mixed-replace message have the same semantic meaning. However, each part invalidates - "replaces" - the previous parts as soon as it is received completely. Clients should process the individual parts as soon as they arrive and should not wait for the whole message to finish.
Originally developed by Netscape,[6] it is still supported by Mozilla, Firefox, Chrome,[7] Safari (but not in Safari on the iPhone) and Opera, but traditionally ignored by Microsoft. It is commonly used in IP cameras as the MIME type for MJPEG streams.[8]
The multipart/byteranges is used to represent noncontiguous byte ranges of a single message. It is used by HTTP when a server returns multiple byte ranges and is defined in RFC 2616.
mime.types