The ornamental design for a convertibleseat, as shown and described.
FIG. 1 is an upper perspective view of the convertible seat showing my design.
FIG. 2 is a front elevation view thereof;
FIG. 3 is back elevation view thereof;
FIG. 4 is a side elevation view thereof;
FIG. 5 is an opposite side elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is an upper perspective view of the convertible seat shown in a folded configuration; and,
FIG. 9 is a side elevation view of the convertible seat shown in a condition of use.
The person presented in broken lines in FIG. 9 is for the purpose of illustrating the use of the convertible seat and forms no part of the claimed design.