mailx não pode enviar anexos e você não tem uuencode. Se você tem mime-construct, então é relativamente fácil:
mime-construct --output \
--to "bernhard@localhost" \
--file-attach fish.zip \
--string here_you_are
a saída:
To: bernhard@localhost
MIME-Version: 1.0 (mime-construct 1.9)
Content-Type: multipart/mixed; boundary=congratulations
--congratulations
Content-Disposition: attachment; filename=fish.zip
Content-Type: application/zip; name=fish.zip
Content-Transfer-Encoding: base64
UEsDBBQAAgAIADKluEK3tLL0XAAAAG8AAAAEABUAZmlzaFVUCQADYLSfUSKiAVNVeAQAAAAAAC2K
wQ2AIBAE/1axDSB8tBMLIHDoJXIQOSV2ryb+JjPDHZEu9N2LA4vSkXwg+BiRuG0OeldCLsJaDqTd
rw1ShAYNNZ65wvA/moZ5cu6FVy4wHbCaq/0qyzp+9/AAUEsBAhcDFAACAAgAMqW4Qre0svRcAAAA
bwAAAAQADQAAAAAAAQAAAKSBAAAAAGZpc2hVVAUAA2C0n1FVeAAAUEsFBgAAAAABAAEAPwAAAJMA
AAAAAA==
--congratulations
Content-Transfer-Encoding: base64
aGVyZV95b3VfYXJl
--congratulations--
junto com mailx:
mime-contruct --your-options | mailx -s your_subject youruser@domain