I had been uploading them using the cp command from my dev directory on my workstation, via the webdav connection I maintain between my workstation and the nextcloud instance on my NAS.
I just tried uploading a file named interfaces.inc using the browser interface. The upload failed and a small popup (that looked like a tooltip) came up for about 5 seconds that said “forbidden”.
The permissions on interfaces.inc are my ordinary user permissions, and they match the permissions specified for the directories accessible through the webdav connection (which is specified like this: sudo mount -t davfs2 https://192.168.0.27:12080/nextcloud/remote.php/webdav/ /home/jiml/owncloud -o rw,uid=jiml,gid=jiml).
So, the file is being blacklisted and - based on what I see - it is being blacklisted based upon suffix. There are other PHP files that are identified as .php which are present in the subdirectory, and in other subdirectories which are devoted to C code, everything is there.
Subsequent to this problem occurring, I did enable the file access control plugin hoping it would help, but it seems this plugin specifically permits me to deny file types, it does not permit me to permit file types that are otherwise blocked.
This particular installation of nextcloud started as an owncloud installation about three years ago, and has been upgraded ever since. Currently it is nextcloud 11.0.5.