The ornamental design for a filter, as shown and described.
FIG. 1 is a perspective view from the side and above of the filter of this invention, the lower face of the filter being disclaimed; and,
FIG. 2 is a side elevational view of the filter, all side-elevational views of the filter being identical.
A filter has a cylindrical outer surface, a planar upper surface, and a central vertically throughgoing hole opening at the top surface at a beveled edge. The filter is rotation-symmetrical about the center axis of the central hole.