在最新的文章啊f uptime-kuma there was a resolution for an issue by erasing a .type file in a container directory.
Based on this I have tried to see the options of sftp and lftp (lftp has great auto-completion and couple nice tools like cat) and it seems that you can actually manage the FS of both RouterOS and their containers data files.
This is not possible using winbox "Files" menu and it's pretty useful to debug and fix containers.
The .type file contains "container" and there for let the "Files" browser know that it should not display and browse this folder (and I fully understand why...)

I didn't managed to change ownership of the files and it's a shame but it's better then nothing.