[FIX] base: don't overwrite existing extensions
[FIX] base: don't overwrite existing extensions

The point was to fix files attachments without an extension,
potentially appending to existing extensions was not a smart idea.

See #34299, 5eb15c01b556620c77c0f7327494d4bf3117d115

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo