The ornamental design for a backpack with attachedlegstraps and seat cushion, as shown and described.
FIG. 1 is a top front perspective view of a backpack with attached leg straps and seat cushion;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a top elevation view thereof;
FIG. 5 is a bottom elevation view thereof; and,
FIG. 6 is a right side elevation view thereof, the left side elevation view being a mirror image thereof.
The broken lines shown in the drawings depict portions of a backpack with attached leg straps and seat cushion that form no part of the claimed invention.