Simple function that returns human readable representation of a given file size.
console.log(fileSize(50000)); // 50 KB
console.log(fileSize(1843875982)); // 1.8 GB
console.log(fileSize(1843875982, 0)); // 2 GB
console.log(fileSize(1843875982, 3)); // 1.844 GB
console.log(fileSize(24886424)); // 24.9 MB
console.log(fileSize(445)); // 445 B