IMultipart Interface

All multipart MIME entities will have a Content-Type with a media type of "multipart". The most common multipart MIME entity used in email is the "multipart/mixed" entity.

Four (4) initial subtypes were defined in the original MIME specifications: mixed, alternative, digest, and parallel.

The "multipart/mixed" type is a sort of general-purpose container. When used in email, the first entity is typically the "body" of the message while additional entities are most often file attachments.

Speaking of message "bodies", the "multipart/alternative" type is used to offer a list of alternative formats for the main body of the message (usually they will be "text/plain" and "text/html"). These alternatives are in order of increasing faithfulness to the original document (in other words, the last entity will be in a format that, when rendered, will most closely match what the sending client's WYSISYG editor produced).

The "multipart/digest" type will typically contain a digest of MIME messages and is most commonly used by mailing-list software.

The "multipart/parallel" type contains entities that are all meant to be shown (or heard) in parallel.

Another commonly used type is the "multipart/related" type which contains, as one might expect, inter-related MIME parts which typically reference each other via URIs based on the Content-Id and/or Content-Location headers.

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