Minecraft wiki entry for Mooshroom, associated text: 0–2 Leather. The maximum amount is increased by 1 per level of Looting, for a maximum of 0-5 with Looting III.
1–3 Raw Beef (Steak if killed while on fire). The maximum amount is increased by 1 per level of Looting, for a maximum of 1-6 with Looting III.
1–3 (when killed by a player or tamed wolf)