For someone who want to use onchange event directly on file input, set onchange="somefunction(), example code from the link:
<html>
<body>
<script language="JavaScript">
function inform(){
document.form1.msg.value = "Filename has been changed";
}
</script>
<form name="form1">
Please choose a file.
<input type="file" name="uploadbox" size="35" onChange='inform()'>
<br><br>
Message:
<input type="text" name="msg" size="40">
</form>
</body>
</html>