For the same file with 1 header line and 3 data fields:

$ ./mongoimport --type csv --headerline -c test -d test --drop /tmp/test.csv
connected to: 127.0.0.1
dropping: test.test
imported 4 objects

$ ./mongoimport --type csv -c test -d test --drop /tmp/test.csv --fields foo,bar,baz,qux
connected to: 127.0.0.1
dropping: test.test
imported 4 objects