Mongoexport --fieldFile error: Failed: open fields.txt

I want more clarification from your end. The following command
mongoexport --db=users --collection=contacts --type=csv --out=C:\opt\backups\contacts.csv will give the error message : Failed: CSV mode requires a field list.
Also, the following command
mongoexport --db=users --collection=contacts --type=csv --fieldFile=fields.txt --out=C:\opt\backups\contacts.csv will give error message : Failed: open fields.txt. The system can not find the file specified.

So, to get an output :
name
address
what mongoexport command should we need ? Please clarify.