claws-mail: fix attachment MIME type detection

Adds support for shared-mime-info to Claws, to fix attachments in
outgoing messages always having MIME type application/octet-stream
because Claws doesn't know where to look, instead complaining:

/nix/store/...-claws-mail-3.11.1/etc/mime.types: fopen: No such file or directory

Moreover, Claws relies on incoming MIME types for knowing when e.g. to
display an attached image, so sending application/octet-stream
unnecessarily is bad.

Tested against release-15.09.