Also having this issue. The mattermost directory and all subdirectories show mattermost as the owner (and re-ran chown -R mattermost just in case). Permissions are set as:
drwxr-x— 3 mattermost mattermost 4,0K Jun 26 07:38 20230626
Tried disabling and re-enabling file sharing in the console, but still not happening – uploads just hang on “Processing…”