Catalyzes the hydrolysis of 10-formyltetrahydrofolate (formyl-FH4) to formate and tetrahydrofolate (FH4). (6S)-10-formyltetrahydrofolate + H2O = (6S)-5,6,7,8-tetrahydrofolate + formate + H(+) Purine metabolism; IMP biosynthesis via de novo pathway; formate from 10-formyl-5,6,7,8-tetrahydrofolate: step 1/1. Belongs to the PurU family.