Content is text. Ofcourse you can serialize other types/sturctures to JSON/XML/etc. to pass them as text.
Type is an optional field used to categorise your message, so that in <On Message> you can handle different message content correctly, e.g. content might contain a (serialized) structure A or structure B, and by checking type, your model can parse it correctly.
Timestamp is currently overriden to specify the time the message was actually published by the server. See bug #341.
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