[Update]
I've managed to upload a small file (but not yet a large image). ../media/filer_public/ sub-directories are being correctly created and file correctly uploaded. Need to investigate nginx configurations.
[OP]
I've logged into a new Django CMS system as superuser but cannot Add filer image or Add filer file to a page as the file upload silently fails; very briefly flashing its upload graphics but not actually uploading anything. I believe all the settings.py are correct as static artifacts are rendered correctly and Nginx has credible similar locations for both media and static directories.
I believe all file and directory permissions and ownerships are correct; i.e. that Nginx has user and / or group ownership of the Django CMS app directories and that permissions are correct.
The Postgres table filer_folder has a row for a new filer folder I created when editing a page but no corresponding directory has been created in the file system. I can add text and new text block plugins that get saved correctly.
Django CMS is running in a Docker container web which I have confirmed has rw (read/write) access to a Docker volume.
I see nothing abnormal in webs logs.
How can I find out what's (not) happening?
Simply adding client_max_body_size 10M; to the nginx configuration for the site solved the issue.
Similar issues were addressed in Stackoverflow and elsewhere: