Bug 840069 [Messaging:MMS] Add Attachment constructor to hold data and render display element. Manage attachments in ThreadUI.compose, retreive message content as an array of text and attachments, and exposeinterface to attach. Unit tests included.
Bug 840069 [Messaging:MMS] Change append/attach to three methods (`insert`, `append` and `prepend`) that can take html, a dom node, or an attachment and insert it at the beginning, end, or at the current caret. Update and improve test coverage.