Multipart Bodies
A MIME part body may contain nested MIME parts. In this case, each nested part contains its own set of headers and its own part body. The top-level part has the media type
multipart
.
The MIME specification defines four multipart subtypes. The MIME package also supports an additional multipart subtype, the
multipart/related
content type described in RFC 2387. All multipart types require that the header value contain a
boundary
parameter. The value of the
boundary
parameter delimits the parts of the message. For example, the message snippet below contains two parts delimited by
__the_boundary__
:
Content-Type: multipart/mixed; boundary="__the_boundary__"
--__the_boundary__
This is the first part, in plain text.
--__the_boundary__
Content-Type: text/plain; charset=us-ascii
This is the second part, also in plain text but with a
Content-Type header.
--__the_boundary__--
Since the MIME format uses the value of the boundary parameter to tell where parts begin and end, it’s important that the value does not occur in the content of any message part. This restriction includes parts which are themselves multiparts. In other words, if a message contains more than one multipart, each multipart in the message must use a different boundary. Otherwise, a recipient has no way to reconstruct the message accurately.
NOTE:
An
RWMimeMultipart
must not contain itself, either directly or indirectly. A part that contains itself cannot be represented as a string.
The
RWMimeUtils
class provides a static
getUniqueBoundary()
function that generates strings unlikely to occur in message content.
The relationship between the parts of a
multipart
media type determines the subtype. The table below describe common
multipart
types:
Table 2 –
MIME multipart subtypes
|
Multipart subtype
|
Relationship of parts
|
|
mixed
|
Parts are unrelated documents (for example, email attachments).
|
|
related
|
Parts are components of a single document (for example, an HTML page and the images in that page).
|
|
parallel
|
Parts should be displayed simultaneously (for example, audio and video).
|
|
alternative
|
Parts contain the same content in alternative formats (for example, an HTML page and a plain text equivalent).
|
|
digest
|
Parts are RFC 822 (email) messages.
|
A
Content-Type
of
multipart/mixed
is typically used for email attachments, and is therefore the most common multipart type. A default-constructed
R-WMimeMultipartType
object represents a
multipart/mixed
header value.
The value of a
multipart/related
Content-Type
contains a
boundary
parameter, a required
type
parameter and two optional parameters,
start
and
start-info
. The
type
parameter matches the
Content-Type
of the part to be processed first. By default, an application begins processing a
multipart/related
body at the first contained part. If the
start
parameter is present, an application begins processing a
multipart/related
body at the part that contains a
Content-ID
value matching the value of the
start
parameter. If the
start-info
parameter is present, the value of the parameter provides information for the processing application. The MIME package provides class
RWMimeMultipartRelatedType
for convenience in working with the additional parameters in a
multipart/related
content type.
