set onloadstart (fn) {
    webidl.brandCheck(this, FileReader)

    if (this[kEvents].loadstart) {
      this.removeEventListener('loadstart', this[kEvents].loadstart)
    }

    if (typeof fn === 'function') {
      this[kEvents].loadstart = fn
      this.addEventListener('loadstart', fn)
    } else {
      this[kEvents].loadstart = null
    }
  }
@see https://encoding.spec.whatwg.org/#bom-sniff
@param {Uint8Array} ioQueue