Blob.type

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

// 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;
  }

  if (Array.from(files).every((file) => allowedFileTypes.includes(file.type))) {
    output.innerText = 'All files clear!';
  } else {
    output.innerText = 'Please choose image files only.';
  }
});