Database
[{_id:ObjectId("637f20ce6ce5c48d9788a1ff"),title:"TEST",files:[{contentType:"application/pdf",name:"imId1",_id:"id1"},{contentType:"application/pdf",name:"imId2",_id:"id2"}]}]
Query
db.collection.aggregate([{"$unwind":"$files"},{$match:{"files._id":"id2"}},{"$replaceRoot":{"newRoot":"$files"}}])