JSFiddle - React, Tailwind, and code Playground

HTML

<input type="file" id="test">
<input type="button" id="check" value="ファイルタイプ確認">

JavaScript

var test = document.getElementById('test');
var check = document.getElementById('check');
check.addEventListener('click',function(e){
  alert(test.files[0].type);
})