Attachment related warnings in browser when upgrading to XWiki 13.1

STEPS TO REPRODUCE

1. Create (or unzip) an older instance of XWiki (e.g. 12.6.8)
2. Close the instance
3. Go to an instance of 13.1 and copy/cut the

   [...]webapps\xwiki

   folder and replace it (first delete the old one) in the same location on the older (12.6.8) version

4. Start XWiki and follow DW

EXPECTED RESULTS

No warnings/errors are displayed in browser when running Distribution Wizard.

ACTUAL RESULTS

On 'Step 2 - Flavor', there are some warnings displayed, related to attachments. The same warnings reproduce when running the DW for a subwiki:

The issue could not be reproduced when upgrading from XWiki 12.6.8 to XWiki 13.0.