The ornamental design for a toolholder accessory, as shown and described.
FIG. 1 is a front, right, top isometric view of a tool holder accessory.
FIG. 2 is a front, right, bottom isometric view thereof.
FIG. 3 is a front elevational view thereof.
FIG. 4 is a rear elevational view thereof.
FIG. 5 is a left side elevational view thereof.
FIG. 6 is a right side elevational view thereof.
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
All broken lines shown herein are representative of an environment and are for illustrative purposes only; the broken lines form no part of the claimed design.