We claim the ornamental design for a filter, as shown and described.
FIG. 1 is a perspective view of a filter showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a side view thereof;
FIG. 5 is a side view from the side opposite that of FIG. 4;
FIG. 6 is a plan view thereof from above; and,
FIG. 7 is a plan view thereof from below.
The dash dot lines immediately adjacent to the claimed areas in FIG. 1 represent the boundaries of the claimed design and form no part of the claimed design.