[dmitriymar]

<form>
  <span id="fileinputplace"><input type="file" required="required" name="file_name" id="username" size="26" onChange="proverka(this)"</span>
</form>
<script>
function proverka(elem)
 {
  if (elem.value.match(/^[^\.]+\.doc(x)?$/i)) alert('true')
  else
   {
     alert("error");
     elem.value="";
    }
   
}
</script>