The actual Mongo output tool does not allow to specify field type (except for the primary _id field). The tool just assumes that all fields are string. Many of our CSV files incorporate string representation of ObjectId (ex: "56df422c08420b523aa00a77").
When we import those CSV we have to run a an additional script that will convert all the id into ObjectId fields. Same thing with the date -> Mongo Date.
If the tool would allow us to do this, it would save us a billion time across all our ETL processes.
Regards,