The ornamental design for a treatmentattachment, as shown and described.
FIG. 1 is a bottom isometric view of a treatment attachment shown the powered treatment device in an environment illustrating my design;
FIG. 2 is a top isometric view showing the design;
FIG. 3 is a right view showing the design;
FIG. 4 is a left view showing the design;
FIG. 5 is a front view showing the design;
FIG. 6 is a back view showing the design;
FIG. 7 is a top view showing the design;
FIG. 8 is a bottom view showing the design; and,
FIG. 9 is an alternate bottom isometric view showing the design.
The broken lines shown in the figures are included for the purpose of illustrating portions of the treatment attachment and form no part of the claimed design.