The ornamental design for a playmat with a pillow border, as shown and described.
FIG. 1 is a top front perspective view of the play mat with the pillow border, showing my new design;
FIG. 2 is a front side elevation view thereof, with the rear side elevation view being a mirror image thereof;
FIG. 3 is a right side elevation view thereof, with the left side elevation view being a mirror image thereof;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The play mat with the pillow border is shown with symbolic breaks in its length and width. The appearance of any portion of the play mat with the pillow border between the break lines forms no part of the claimed design.