Purpose: Before it was hard to view expense attachments from sheet record.
To do so, the user should have gone through clicking on each expense first and
checking attachments, or could have clicked on a attachments smart button on embedded
expense table, which is redirecting to another view that only displays attachments for the one expense only.
Thus, checking all the attachments from sheet was an tedious task.
After this commit, the user can see in the sheet attachment preview all the associated expenses' attachments.
To make it possible, we ovveride 'mail.thread' method that is called to
fetch the data for the chatter. In the override, we fetch extra, associated expenses' attachments.
task - 2320177
closesodoo/odoo#92724
Signed-off-by: Kevin Baptiste <kba@odoo.com>