The ornamental design for a filter, as shown and described.
FIG. 1 is a top, front, left-side perspective view of the filter;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a right-side elevational view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a cross-sectional view thereof taken along line 6-6 in FIG. 2;
FIG. 7 is a cross-sectional view thereof taken along line 7-7 in FIG. 3; and,
FIG. 8 is a cross-sectional view thereof taken along line 8-8 in FIG. 3.
The left-side elevational view is a mirror image of the right-side elevational view, and the rear elevational view is a mirror image of the front elevational view. Thus, the left-side elevational view and the rear elevational view are not provided.