HTMLInputElement.multiple

// fileInput is a <input type=file multiple>
let fileInput = document.getElementById("myfileinput");

if (fileInput.multiple == true) {
  for (let i = 0; i < fileInput.files.length; i++) {
    // Loop fileInput.files
  }

  // Only one file available
} else {
  let file = fileInput.files.item(0);
}