I claim the ornamental design for a hair dryer attachment, as shown and described.
FIG. 1 is a top perspective view of a hair dryer attachment;
FIG. 2 is a right side view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a front side view thereof; and,
FIG. 7 is a back side view thereof.
The broken lines in the drawings of the hair dryer attachment are for showing the clips of the hair dryer attachment and form no part of the claimed design.