I solved the problem by deleting the file "/var/www/nextcloud/core/js/mimetypelist.js from my installation and I replaced it with the file recovered from the latest stable version of nextcloud (https://download.nextcloud.com/server/releases/latest.zip).
After that the error disappeared.
Don’t forget to restore the correct rights to the file,
chown -R www-data:www-data /var/www/nextcloud/core/js/mimetypelist.js
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