RFC1521, the Multipurpose Internet Mail Extensions memo, defines a
structure which is the base for all messages read and written by
modern mail and news programs. It is also partly the base for the
HTTP protocol. Just like RFC822, MIME declares that a message should
consist of two entities, the headers and the body. In addition, the
following properties are given to these two entities:
- Headers
A MIME-Version header must be present to signal MIME compatibility
A Content-Type header should be present to describe the nature of
the data in the message body. Seven major types are defined, and an
extensive number of subtypes are available. The header can also
contain attributes specific to the type and subtype.
A Content-Transfer-Encoding may be present to notify that the data
of the body is encoded in some particular encoding.
- Body
The MIME module can extract and analyze these two entities from a stream
of bytes. It can also recreate such a stream from these entities.
To encapsulate the headers and body entities, the class MIME.Message is
used. An object of this class holds all the headers as a mapping from
string to string, and it is possible to obtain the body data in either
raw or encoded form as a string. Common attributes such as message type
and text char set are also extracted into separate variables for easy
access.
The Message class does not make any interpretation of the body
data, unless the content type is multipart. A multipart
message contains several individual messages separated by boundary
strings. The Message->create method of the Message class will
divide a multipart body on these boundaries, and then create
individual Message objects for each part. These objects will be
collected in the array Message->body_parts within the original
Message object. If any of the new Message objects have a body of
type multipart, the process is of course repeated recursively.
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