html

<input type="file" id="input" multiple />
<output id="output">Choose image files…</output>

js

// Our application only allows GIF, PNG, and JPEG images
const allowedFileTypes = ["image/png", "image/jpeg", "image/gif"];

const input = document.getElementById("input");
const output = document.getElementById("output");

input.addEventListener("change", (event) => {
  const files = event.target.files;

  if (files.length === 0) {
    output.innerText = "Choose image files…";
    return;
  }

  const allAllowed = Array.from(files).every((file) =>
    allowedFileTypes.includes(file.type),
  );
  output.innerText = allAllowed
    ? "All files clear!"
    : "Please choose image files only.";
});