Toner container and image forming system

An attachment for being mounted to an image forming apparatus, the attachment comprising:

• a projection having an inner peripheral surface centered on a central axis and projecting in a direction of the central axis outside the inner peripheral surface in a radial direction of an imaginary circle centered on the central axis,
• wherein, when the attachment is oriented in a predetermined direction in which the central axis extends in a direction of gravity and the projection projects downward, (i) the projection has a first downward surface facing downward, a second downward surface facing downward, and an upward surface facing upward, (ii) when a circumferential direction of the imaginary circle is a first circumferential direction, and a circumferential direction opposite to the first circumferential direction is a second circumferential direction, the first downward surface and the second downward surface extend upward in the first circumferential direction, and at least a part of the first downward surface is at a position which is closer to the central axis in the radial direction than the second downward surface is to the central axis and which is different from a position at which the second downward surface is provided in the circumferential direction, and (iii) at least a part of the upward surface is above at least a part of the second downward surface.