About that, what, which

the list is for A that i sent you before. (O)
the list is for A which i sent you before.(O)
the list is for A what i sent you before. (X)
the list is for A (skip)i sent you before. (O)

(Explanation 1)

▲ i sent   you             ∧         before.
      (sent -to whom   -object   before)
   
because we delete the conjunction and object that will be in  ∧ between the first sentence and the second sentece, the objective relative pronoun in ▲ is 'which' or 'that' (When an antecedent is not a person.)

(Explanation 2)

the list is for A what i sent you before. (X)
antecedent A + relative pronoun in Explanation 1 (which or that) ==> What
An antecedent can't be prior to relative pronoun 'What'

By Raven