codice:
<form name='myform' onSubmit="return control();" method='post'>
<input type="file" name="my_file">


<button type="submit" name="crea" id="upload" class='btn btn-success'>Inserisci</button>


</form>

<script>
function control(){
 var filea=document.myform.my_file.value;
  if(filea==""){
    alert("Selezionare un file");
return false;
}else{alert('file inviato');}

}
</script>