The ornamental design for a foldableshelf, as shown and described.
FIG. 1 is a left side elevation view of a foldable shelf in the unfolded position showing my design; the right side elevation view being a mirror image thereof;
FIG. 2 is front elevation view thereof;
FIG. 3 is top plan view thereof;
FIG. 4 is back elevation view thereof;
FIG. 5 is a perspective view of a foldable shelf in the unfolded position showing my design;
FIG. 6 is a side view perspective of a foldable shelf in the folded position showing my design;
FIG. 7 is a left side elevation view thereof, the right side elevation view being a mirror image thereof; and,
FIG. 8 is an upper perspective view of a foldable shelf in the folded position showing my design.