The ornamental design for a topsurface of a floor mat, as shown and described.
FIG. 1 is a top plan view of a first embodiment of the top surface of a
floor mat according to our novel design; and,
FIG. 2 is a top plan view of a second embodiment of the top surface of a
floor mat according to our novel design, the sole difference from the
first embodiment resides in the opposite alternating pattern of light to
dark areas.