I need to prevent upload of .CDX files to Nextcloud 15 (snap version)

On my laptop I have a FoxPro app which I connect to my server (PC) remotely. Each time I login from my laptop app to the master db on the PC, Nextcloud tries to upload .CDX files which contains indexes to the .dbf files and instead it ends up conflicting those files.
I need to restrict upload for the .cdx files from the server.
I have the File Access Control app setup on the cloud server and the archive files are restricted correctly (rar, zip and tgz files are not uploaded). /^application/(zip|x-zip-compressed)$/i is used as an example.
Now, the .cdx files are “chemical” mime type files, and I tried a lot of combinations in order to prevent uploading this type of files. Nothing works. /^chemical/(cdx|x-cdx-chemical)$/i is used. I created copies from the .dist.json mimetype files and placed them inside the /config/ folder on the server, but, when I try to sudo -u www-data php nextcloud.occ maintenance:mimetype:update-js the server returns "Apps directory not found! error… I mentioned in the subject that the server is a snap version of Nextcloud 15. I tried everything from the manual. Not even maintainance mode off is working. The same error appears. And the apps folder is inside /snap/nextcloud/current/htdocs/ where it should be… What am I missing here?