An attaché is a thin briefcase used for carrying papers. It is sometimes called an attaché case. You may notice that in spy movies, an attaché is often swapped out by the bad guy, and then the good guy spends the rest of the movie trying to get it back.
Attaché comes from the French attacher, which literally means "attached," and it was first applied to someone working for a diplomat, such as a junior officer. Now an attaché is a technical expert assigned to the staff of a diplomatic mission. If you are an expert on the customs of a certain country, you may be asked to be a cultural attaché and advice the ambassador for that country.