Before, it was shown by UI when the chat is empty, however, in case of
verified groups, this is never the case any longer as the 'e2ee now
guaranteed' is always added.
It can be used e.g. as a default in the file saving dialog. Also display the original filename in
the message info. For these purposes add Param::Filename in addition to Param::File and use it as an
attachment filename in sent emails.