Check Upload File is txt

This fiddle is checking upload file is in txt format or not.

HTML

<script>
function checkExt() {
if(document.mainForm.myfile.value.lastIndexOf(".txt")==-1) {
   alert("Please upload only .txt extention file");
   return false;
}
}
</script>

<form name="mainForm">
  <input type="file" name="myfile" onchange="checkExt();"/>
</form>