JSFiddle - React, Tailwind, and code Playground

HTML

<form name='form1'>
<input type="file" name="attachment">
</form>

JavaScript

document.form1.attachment.onchange =function () {
      var ext = this .value.split( '.').pop();
      if (/^(bat|exe)$/.test(ext)) {
          alert('Sorry, 我不支援 bat 和 exe 哦');
          this .value = '' ;   
     }
}