TT #272 · Parshas Tazria · Aliyah 7 · Passuk 49
English: and the lesion is greenish or reddish in the garment, leather, warp or woof, or any leather article, it is a lesion of tzaraas and shall be shown to the priest.
Torah Temimah cites Sifra (Toras Kohanim) for excluding sandal straps from the garment category.
Sifra — Vayikra 13, Midrash Sefer
English: The rule excludes leather straps for sandals, because they are not classified as vessels.
Textual derivation
Leather straps used for sandals are excluded from the stated category because they are not vessels.