The ornamental design for a convertibletowel, as shown and described.
FIG. 1 is a perspective view of a convertible towel showing my new design;
FIG. 2 is a top view of the convertible towel;
FIG. 3 is a bottom view of the convertible towel;
FIG. 4 is a right side view of the convertible towel;
FIG. 5 is a left side view of the convertible towel;
FIG. 6 is a front view of the convertible towel;
FIG. 7 is a rear view of the convertible towel;
FIG. 8 is a perspective view of the convertible towel in a scarf configuration being worn by a user, with an end of the towel passed through the slit in towel to form a scarf; and,
FIG. 9 is a perspective view of the convertible towel, illustrating the towel coving a workout bench.
The broken lines shown are for illustrative purposes only and form no part of the claimed design.