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 features drawn in broken lines depict portions of the mortarboard attachment that form 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.