People use the word attache and briefcase as referring to the same thing and it was not so historically.
an attache briefcase was always a structured box made from metal or wood and usually wrapped in leather. It had no strap and just a handle with locks. Many of the high-end attache briefcases were made from English bridle leather and lined in leather or suede. James Bond was seen carrying a black bridle leather attache case in the movie From Russia With Love. The attache briefcase is great if you have items that you do not want to be crushed by the leather or canvas bag. It is usually heavier than leather or canvas but some of the aluminum ones are even lighter than a leather briefcase.
traditionally a briefcase was made from leather or canvas and had a handle with no shoulder strap. The mail bag or satchel usually had a shoulder strap and no handle and then this morphed into the messenger bag. Today most brands and makers use all these words to mean different things. A bridle leather briefcase is not as soft as a chromium tanned leather briefcase and not as hard as an attache. It gives you the rigidness of a wood box but not as heavy. A briefcase is also much more comfortable to carry on a shoulder strap.