Action Speed Buffs

Imagine you have 50% action speed. Then wizards buffs you for additional 10% action speed. Is it considered flat raise (e.g. 50% + 10% = 60%, so new animation length is 1 / (1 + 0.6) = 0.625) or it is a multiplier (e.g. 50% * 1.1 = 55%, so new animation length is 1 / (1 + 0.55) = 0.645)?