To increase the capacity of an extension attachment for a bulk material container comprising a frame (32) and a plurality of flaps (33, 34) linked to the frame (32), which are capable of swivelling and, in a swivelled-out position, surround an internal space (31) of the extension attachment, the internal surface (28) facing the internal space (31) of at least of one of the flaps (33) has a concave shape in the swivelled-out position.