The ornamental design for a brushattachment for a flat iron, as shown and described.
FIG. 1 is a side perspective view of a new, ornamental design for a brush attachment for a flat iron with the brush attached to the flat iron;
FIG. 2 is a side perspective view of the brush detached from the flat iron;
FIG. 3 is an enlarged bottom perspective view of the brush attachment when detached from the flat iron;
FIG. 4 is a first side elevation view of the brush attachment for a flat iron with the brush attached to the flat iron;
FIG. 5 is a second side elevation view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is an enlarged front end view thereof; and,
FIG. 9 is an enlarged rear end view thereof.
The broken line showing of a flat iron is included for the purpose of showing environmental context for the brush attachment for a flat iron and form no part of the claimed design.