use insertIfNotExists to store new mimetypes.
use insertIfNotExists to store new mimetypes.

Also throw an error if we can't find the mimetype after insert

Signed-off-by: Robin Appelman <474ee9ee179b0ecf0bc27408079a0b15eda4c99d@icewind.nl>

andreas-p/nextcloud-server,nextcloud/server,nextcloud/server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,nextcloud/server,andreas-p/nextcloud-server,andreas-p/nextcloud-server,nextcloud/server