I even tried to attach a .txt file and sent it to my mail and then downloaded the attachment. The attachment is clearly visible in the Documents folder (in Filebox) but not visible in the Documents app.
As far as the "acmcli" command I really don't know how to use it but as I said I also used the system way of importing my certificate (Settins-Security-Certificates) and it shows that the certificate is imported, but it does not work. I am not sure if Firefox can use the certificates imported in the system or they have to be imported in Firefox as well?