On windows there is no default application for .mbox or .eml files. And even so an "open with kleopatra" entry makes sense for me to add.
This will mean that kleopatra will be called like "kleopatra /tmp/foo.mbox".
With mimetreeparser-widgets example this works. I just wonder how to display the results. Because the decryption results are much better displayed in our resultlist then in the mimetreeparser-widgets example.