There is no entry for application/x-go-sgf, hence, due to glob-patterns
sgf-files are assigned mimetype application/x-tex-gf

  mime/packages/freedesktop.org.xml
  2174    <mime-type type="application/x-tex-gf">
  2175      <comment>generic font file</comment>
  2176      <comment xml:lang="no">vanlig skriftfil</comment>
  2177      <glob pattern="*.*gf" />
  2178    </mime-type>

application/x-go-sgf refers to 'Smart Game Format'
(http://www.red-bean.com/sgf/)  and should probably be described as

    <mime-type type="application/x-go-sgf">
      <comment>Smart Game Format</comment>
      <glob pattern="*.sgf" />
    </mime-type>

There actually seem to be quite a few other formats named sgf, but I do think
that 'Smart Game Format' is the most common. I'm not sure, but other mime-types
for the same format may be available (since other viewers most probably is
desireable for different game-types), such as
  application/x-hex-sgf
  application/x-backgammon-sgf
however, google turns up with zero hits and so far the only applications using
sgf to store data are go-applications.