"File" is emphatically not neerzetten in Dutch. Neerzetten is only used for the act of physically putting something somewhere, i.e. putting a vase on the table (de vaas op de tafel neerzetten). The verb you're looking for here is archiveren or classificeren, possibly even rubriceren. The sentence would then be, Archiveer/Classificeer dit onder 'B'. It sounds weird, though. I'd say, Dit valt onder categorie 'B'.