The ornamental design for a writinginstrumentattachment, as shown and described.
FIG. 1 is a perspective view of a writing instrument attachment in accordance with the present invention;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof; and,
FIG. 6 is a left side view thereof, the right side view being a mirror image.
The broken lines showing the character “A” in the FIGS. illustrate portions of the writing instrument attachment that form no part of the claimed design.