The ornamental design for an attachment for a cap, as shown and described.
FIG. 1 is a front perspective view of an attachment for a cap showing our new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a right side elevation view thereof,
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is a front perspective view thereof shown in the environment of a cap.
The broken line outline of the cap in FIG. 9 depicts environmental structure of the attachment for a cap and forms no part of the claimed design. The dot-dash broken lines represent boundaries of the claimed design and form no part of the claimed design. All other broken lines in the drawings depict portions of the attachment for a cap that form no part of the claimed design.