The ornamental design for air treatment system filter, as shown and described.
FIG. 1 is a front perspective view of the air treatment system filter of the new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a back elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The claimed portions of the various embodiments of the air treatment system filter are shaded. FIGS. 1-7 show features of the air treatment system filter in broken lines. These broken lines depict environmental subject matter only and form no part of the claimed design. Further, the broken lines immediately adjacent the solid-line portions of the design form the boundary of the design, with the broken lines forming no part of the claimed design.