When Thor is detained, Loki briefly appears wearing a jacket, dress shirt, tie, scarf and long overcoat:
Crazy hi-def trailer screencaps:
For the brief sequence in Stuttgart, Loki is wearing a black suit & tie with a scarf and an overcoat or frock coat. The following cropped screencaps have been lightened to better show the details of the costume:
Note that Loki is wearing highly polished black dress shoes with laces. His white dress shirt alternates from being a plain white shirt to being a ribbed, tuxedo-style shirt. Note also the subtle, ‘scaly’ pattern on the necktie.