I don't like this too much, because a real mms message from the API will have a sequence of this: https://mxr.mozilla.org/mozilla-central/source/dom/webidl/MozMmsMessage.webidl#8 in this property. So I fear we can have issues later.

I think we need to use either another property, or keep the call to Compose.append in ActivityHandler for now, until we revisit this.