The ornamental design for a frontbumper for an automobile, as shown and described.
FIG. 1 is a front and left side perspective view of a front bumper for an automobile showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a right side elevational view thereof; and,
FIG. 5 is a left side elevational view thereof.
The broken lines shown in the drawings illustrate portions of the front bumper for an automobile that are environmental structures and form no part of the claimed design. The drawings include grayscale surface shading which illustrates contour, not any particular color.