The ornamental design for a filter, as shown and described.
FIG. 1 is a front perspective view of a filter showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left-side elevational view thereof;
FIG. 5 is a right-side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a rear perspective view thereof.
The ornamental design which is claimed is shown in solid lines in the drawings. Features and portions in broken lines are presented for illustrative purposes and form no part of the claimed design.