A bug in Nextcloud Server 17.0.1 causes the workflow rules to depend their behaviour on the file extension when checking file mimetypes.
https://hackerone.com/reports/697959
http://lists.opensuse.org/opensuse-security-announce/2020-02/msg00022.html
http://lists.opensuse.org/opensuse-security-announce/2020-02/msg00019.html