The ornamental design for an attachmentadapter, as shown.
FIG. 1 is a front, top and left side perspective view of an attachment adapter showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a left side elevational view thereof;
FIG. 8 is a front, top and left side perspective view showing a state where the attachment adapter is opened; and,
FIG. 9 is a reference perspective view showing a usage state of the attachment adapter (the figure shows a state where a cylindrical-shaped interchangeable lens digital camera as one example shown by the broken lines and a smartphone as one example shown by the broken lines are connected to each other by using the attachment adapter shown by the solid lines).