A rectangle is another French word the English decided to snatch. Rec means droit (straight) in French and angle is the same in French as in English.
Even though you probably hear the word file more often than dossier, it is still a word used in the English language quite often. A dossier in French has the word dos (back in it). You often find labels on the back of dossiers, which is why they called it that in the first place.
Detours can be quite annoying if you’re trying to get somewhere quickly. Unfortunately, we can’t blame road detours on the French even if we borrowed (and never gave back) the word from them. In French, détour comes from détourner which means to deviate from the direct path.