The ornamental design for bag with an adjustablestrap, as shown and described.
FIG. 1 is a perspective view of a bag with an adjustable strap.
FIG. 2 is a left side view of FIG. 1.
FIG. 3 is a right side view of FIG. 1.
FIG. 4 is a front view of FIG. 1.
FIG. 5 is a back view of FIG. 1.
FIG. 6 is a top view of FIG. 1; and,
FIG. 7 is a bottom view of FIG. 1.
The dashed lines in the drawings depict stitching and form a part of the claimed design.
The interior portion of the bag with an adjustable strap shown in broken lines in FIG. 6 forms no part of the claim. The dashed lines on claimed surfaces depict claimed stitching.