The ornamental design for a light fixture with a finial, as shown and described.
FIG. 1 is a top plan view of a light fixture with a finial showing my design;
FIG. 2 is a bottom plan view thereof; and,
FIG. 3 is a side elevation view thereof.
Due to the symmetric nature of the design, the left, right, front and back sides of the light fixture with a finial are identical, and, thus, only one view is shown herein.