The ornamental design for a mortarboardattachment, as shown and described.
FIG. 1 is a top perspective view of a mortarboard attachment showing our new design.
FIG. 2 is a front elevational view of the mortarboard attachment.
FIG. 3 is a back elevational view of the mortarboard attachment.
FIG. 4 is a top plan view of the mortarboard attachment.
FIG. 5 is a bottom plan view of the mortarboard attachment.
FIG. 6 is a left side view of the mortarboard attachment; and,
FIG. 7 is a right side view of the mortarboard attachment.
The rectangular feature drawn in broken lines depicts a portion of the mortarboard attachment that forms no part of the claimed design. All other broken lines which illustrate a mortarboard cap depict environmental structure that forms no part of the claimed design.