JSFiddle - React, Tailwind, and code Playground
HTML
<form>
<input type="file" name="files" id="file1" style="color:White" />
<input type="submit" value="Create" />
</form>
<button value="check" id="check" name="check" >check</button>
JavaScript
$("#check").click(function(){
var fileName = $("#file1").val();
if(fileName.lastIndexOf("png")===fileName.length-3)
alert("OK");
else
alert("Not PNG");
})