Hi,
sorry to writing to topic one month old, but i had this problem too and found a solution, so I think it will be handy for someone:
Just run as root:
update-mime /usr/share/mime/
update-mime-database /usr/share/mime/
gtk-update-icon-cache /usr/share/icons/YourTheme
Logout/Login and it will be all right now :-)
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