sendUpdate: talk more about json serialisation, because many more objects can not be serialised, not only binary data, also DOM element objects, Symbols and others.
filter.mimeTypes: file is selectable if either mimetype or file extension matches (made statement bold)
setUpdateListener: elaborate on the undefined behaviour (even different pages or frames is currently undefined behaviour so the per object is in reality rather per app)
attempt to make some things more clear: