A message part with charset "x-unknown" produces the following error in message list view and can not be viewed.

error: could not get structure: unknown charset: unknown charset: charset "x-unknown": htmlindex: invalid encoding name

The source of the message part starts like this

--18446744071601889533-1783259886-1730826805=:24539
Content-Type: TEXT/html; CHARSET=X-UNKNOWN
Content-Transfer-Encoding: BASE64
Content-Description: Muster-Widerrufsformular.html
Content-Disposition: attachment; filename=Muster-Widerrufsformular.html