Cannot export/import all from a fresh XWiki jetty/hsqldb

I just discovered that on a fresh instance of XWiki Jetty/HSQLDB we cannot export and reimport the whole content of the wiki because of the usual:

ERROR 400 Unable to parse form content

This is a known error related to the way we import the pages, opening a new issue since I'm wondering if we should fix this for the LTS by increasing the `maxFormContentSize` by default.

Reproduction step:

• Open a new XWiki Jetty/HSQLDB
• Go to administration -> Export
• Select all pages of the wiki and default options
• Go to administration -> Import
• Upload the backup.xar and try to import the pages with default options

Expected result:

• all pages are imported back

Obtained result:

• the UI display:

  Failed to import documents. Reason: Bad Request
  

• the network shows:

  HTTP ERROR 400 Unable to parse form content