chore: Add an indicator for incoming emails (#1112)

app/views/api/v1/accounts/conversations/messages/index.json.jbuilder

@@ -13,6 +13,7 @@ json.payload do
     json.inbox_id message.inbox_id
     json.conversation_id message.conversation.display_id
     json.message_type message.message_type_before_type_cast
+    json.content_type message.content_type
     json.created_at message.created_at.to_i
     json.private message.private
     json.source_id message.source_id