['a', 'b', 'c'] => "a, b e c" https://repl.it/@lubien/allowedExtensions

allowedExtensions.js

function allowedExtensions (exts) {
  return exts.map((item, i) => {
    const first = i === 0
    const last = i === (exts.length - 1)

    if (first) {
      return [item]
    }

    if (last) {
      return [' e ', item]
    }

    return [', ', item]
  })
  // flatten
  .reduce((acc, x) => acc.concat(x), [])
  .join('')
}

console.log(allowedExtensions(['erick', 'azul', 'amarelo', 'estranhuS']))
console.log(allowedExtensions(['feIo', 'estranhuS']))
console.log(allowedExtensions(['estranhuS']))
console.log(allowedExtensions([]))

Please come home Array#flatMap