We claim the ornamental design for a backpack flap with strap, as shown and described.
FIG. 1 is a perspective view of the backpack flap with strap showing environmental features that include a flap with knives secured in place inside of pouches;
FIG. 2 is a front side view thereof;
FIG. 3 is a rear side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top side view thereof;
FIG. 7 is a bottom side view thereof;
FIG. 8 is a perspective view thereof, which shows the strap fastener partially lifted from the base fastener;
FIG. 9 is a perspective view of an exemplary outer backpack which may be used for the backpack flap with strap;
FIG. 10 is a perspective view of the exemplary backpack in a partially opened state; and,
FIG. 11 is a perspective view of the backpack in a fully opened state of which the backpack flap with strap is a part.
Broken lines, such as those representing other compartments and the outer backpack as a whole, are for environmental purposes only and form no part of the claimed design.