The ornamental design for a doorknobattachment, as shown and described.
FIG. 1 is a front top perspective view of a door knob attachment showing my new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
Those portions of the figures that are presented in broken lines merely illustrate the environment of the design and are not part of the claimed design.