FIG. 1 is a perspective view of a mobile attachment device showing my new design;

FIG. 2 is a top view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is an end view thereof;

FIG. 5 is a left side view thereof; and,

FIG. 6 is a bottom view thereof.

The broken lines in FIGS. 3-6 are for illustrative purposes only and form no part of the claimed design.