Note : When does SN2 attack occur. Let's analyse this and you will get your answer in end.
First of all the steric hindrance around the carbon atom should be least i.e if RX is 1° (primary) then SN2 attack always (mostly) occurs.
If RX is 3° (tertiary) then SN1 attack always (mostly) occurs, as steric hindrance is very high.
Now if RX is 2° (secondary), the mechanism can be SN1 or SN2, depending on the Nucleophile and the solvent. If solvent is aprotic and nucleophile is fairly good (strength): like DMF, acetone etc., then SN2 attack is favoured (I am not explaining why it occurs, but you can refer to the mechanism) If solvent is protic and nucleophile is not so strong: then SN1 mechanism is favoured.
In your question, $\ce{I-}$ (iodide ion) is mild nucleophile and acetone is aprotic solvent. Thus SN2 mechanism is favoured more.
