db={"users":[{_id:"xxx",name:"name",files:[{fileName:"test",size:654},{fileName:"test2",size:50}]}]}

db.users.aggregate([{"$match":{"files.fileName":"test"}},{"$replaceWith":{"$first":{"$filter":{"input":"$files","as":"file","cond":{"$eq":["$$file.fileName","test"]}}}}}])