If Linux Mint 19 (Tara) shows error during update fril 18.3.
Unknown media type in type 'all/all'
Unknown media type in type 'all/allfiles'
To fix run following with sudo user:
cd /usr/share/mime grep -R all/all *
in kde xml, remove the sections containing:
<mime-type type="all/allfiles">
<mime-type type="all/all">
sudo vi packages/kde.xml
the other files showing these sections can remain untouched.
Then run following, no errors should occur
sudo update-mime-database /usr/share/mime
Source: Unknown media type in type Fix