My understanding is that when a house is on fire,

• the water damage from the firefighters cause damage.

• Also the fire is normally “moved” with some fun airflow physics that firefighters learn, so the damage from burning is somewhat contained (hopefully).

• also a fire burns up, so something insulated underneath where the fire is could last a long time with little damage.

Based on these reasons, I expect that the briefcase could help keep your documents safe.