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For a large binary message such as a video or a high resolution photo, a COI-compatible client MAY send add a preview message part. This allows clients with unstable or rate-limited network connections to show the message without needing to download the full message contents first.

A COI-compatible client MUST be able to process such a preview messages part and not show two individual messagesparts. It MAY ignore such messages message parts to be compliant.

A preview message sets the "Chat-Content" header field to "preview" with the message-ID of the large message set as a "reference" parameter in the same header field, for examplerespective part:

 Chat-Content: preview;    reference=<coi$>

A preview message part typically contains one lower resolution image in a single binary message as described above. A preview message SHOULD NOT contain a read receipt request, i.e. no "Disposition-Notification-To" header.

Edit and Delete Messages

To allow users to change already sent messages, a follow up edit message can be send. A COI-compatible client SHOULD only show the latest edited message, but it MAY make the history available. An edit message MAY be empty, this means that the original message should be shown as deleted. A COI client SHOULD visualize the edited state of a message.