The ornamental design for a filter, as shown and described.
FIG. 1 is a perspective view obliquely from the front of an embodiment of a filter, showing my new design;
FIG. 2 is a perspective view obliquely from the rear thereof;
FIG. 3 is a top/bottom view thereof; and,
FIG. 4 is an enlarged top, right view showing a portion of the embodiment of FIG. 2 in more detail.
The evenly spaced broken lines of the filter shown in the drawings depict environmental subject matter and does not form any part of the claimed design. The dot-dash lines show in the drawings depict boundaries and form no part of the claimed design.