The ornamental design for an attachmentdevice, as shown and described.
FIG. 1 is a top perspective view of an attachment device showing my new design.
FIG. 2 is a right side perspective view;
FIG. 3 is a top plan view;
FIG. 4 is a bottom plan view;
FIG. 5 is a left side elevation view;
FIG. 6 is a right side elevation view;
FIG. 7 is a front side elevation view; and,
FIG. 8 is a rear side elevation view.
The broken line showing of a continued length of the item attached to the attachment device is shown for illustrative purposes only and forms no part of the claimed design.