The ornamental design for, a frontbumper for an automobile, as shown and described.
FIG. 1 is a perspective view of a front bumper for an automobile, showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a top plan view thereof; and,
FIG. 4 is a right side elevation view thereof.
The left side elevation view is a mirror image of the right side elevation view.The broken lines are for illustrative purposes only and form no part of the claimed design.