docs: add repair-filecache when mimetype has been updated

Signed-off-by: LouisSung <37973545+LouisSung@users.noreply.github.com>

Author: LouisSung

admin_manual/configuration_mimetypes/index.rst

@@ -21,6 +21,9 @@ command to propagate the changes through the system. This example is for
 Ubuntu Linux::
 
   $ sudo -u www-data php occ maintenance:mimetype:update-js
+
+  # you may also need to update the mimetype for existing files, see nextcloud/server#30566
+  $ sudo -u www-data php occ maintenance:mimetype:update-db --repair-filecache
   
 See :doc:`../configuration_server/occ_command` to learn more about ``occ``.