I'm lacking experience on the following issue and unfortunately it isn't documented explicitly. I'ld like to have a shared files folder for a handful of multi-sites. As those sites all represent sub-organisations of one and the same parent organisation it should be possible that all of them have access to the same files in one certain files subdirectory using a shared file manager like IMCE.

The only concrete clue I found is on https://www.drupal.org/node/53705. But this is about Drupal 5.

A /files directory could easily be shared across two domains without being shared across the remaining domains.

I wildly guess this didn't change much for following versions. But, have anybody ever tried it this way in production? Would you have any concerns doing it this way? What else could I consider to do instead?

As far as I've tested it right now this seems to work. Also images and files uploaded to image and file fields on site A stayed in the shared files folder even after multiple cron runs on site B and site C (as Clive noted to have a look at the file_usage logic).

I also then deleted the files and images from site A's nodes and run site A's cron multiple times. Still the files stayed there. But this simply might be a permission issue, maybe.