Node.js parallel file download, the ES6 way
Things that come to my mind after a quick read:


do not use const someFunction = (...) =&gt; {...} instead of function someFunction(...) {...})
use module.exports even if it is a script
group functionality into functions; you have comments beautifully spaced, but a function name will be more readable and maintainable
try to have functions that do one thing; when explaining to someone what a function does, and you have to use and then you should split the function
try to use map, filter, etc. instead of having iterations with a variable (e.g. fileNumber)
whenever you have someString + someOther you could use string templating (i.e. ${someString}${someOther}
prefer using promises instead of callbacks; the code ends up being more readable; e.g. use bluebird.promisify
use a good eslint config (e.g. airbnb)


To enhance collaboration, you could use a github repository and reference it here.