Filesystem attachments broken in subwikis when upgrading directly from < 9.10 to >= 10.1

STEPS TO REPRODUCE

Preconditions: An instance of XWiki 8.4.6 and an instance of XWiki 10.3 (to which will be migrated) are available.

1. Go to the older instance's folder (e.g. 8.4.6) and change storage configuration to FileSystem by editing the

   [...]webapps\xwiki\WEB-INF\xwiki.cfg

   file

2. Uncomment and modify these parameters as follows:

   xwiki.store.attachment.hint = file
   xwiki.store.attachment.versioning.hint = file
   xwiki.store.attachment.recyclebin.hint = file

3. Delete the 'data' folder (if exists)
4. Start XWiki instance
5. Follow the Distribution Wizard to install the flavor
6. Login as Admin and create a new subwiki
7. Close the instance
8. Go to the newer instance (e.g. 10.3) and copy/cut the

   [...]webapps\xwiki

   folder and replace it (first delete the old one) in the same location on the older version

9. Start the instance and follow the Distribution Wizard through the upgrade process

EXPECTED RESULTS

The upgrade process is completed successfully without errors.

ACTUAL RESULTS

On Step 1 - Flavor, in install log and in console, there are encountered some errors related to failing to load attachments for the subwiki. Please find attached the logs and screenshots.
However, if the instance to be migrated (8.4.6) was created without FileSystem setting enabled, the upgrade process is completed without the respective errors.