The ornamental design for a filebox, as shown and described.
FIG. 1 is a front perspective view of the file box;
FIG. 2 is a front elevation view of the file box;
FIG. 3 is a left side elevation view of the file box;
FIG. 4 is a top plan view of the file box; and,
FIG. 5 is a bottom plan view of the file box.
The broken line showing of the unclaimed part of the file box in all figures is included for the purpose of illustrating environment and forms no part of the claimed design.