This change adds extra diagnostic coverage to help investigate a recurring failure in automated form change handling. It is intended to make the issue easier for developers to identify without changing normal business workflows.

The branching added in #238705 to try and catch https://runbot.odoo.com/odoo/error/234669 red-handed seems to help not at all: the line numbering in https://runbot.odoo.com/runbot/build/95118641 (in the text logs) indicates that it's running with the branching but it still takes the first branch (so it's receiving some sort of `Mapping` for sure) then fails anyway.

This update enhances the visual appeal of spreadsheets by allowing images and graphs to be displayed as thumbnails. This improves the user experience when browsing and selecting spreadsheet templates, making them easier to identify and use. It's a small but impactful improvement to the overall spreadsheet functionality.