ciao a tutti qualcuno sa dirmi perche' questo form non mi manda la variabile file al file prova.php?

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>File sender</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script language="JavaScript" type="text/javascript">
<!--
function checkform ( form )
{
if (form.file.value == "")
{
alert( "Please enter a file." );
form.file.focus();
return false ;
}
return true ;
}
</script>
</head>

<body>
<form name="form1" enctype="multipart/form-data" method="post" action="prova.php" onSubmit="return checkform(this);">
<table width="500" border="0" align="center">
<tr>
<td><div align="center">
<input name="file" type="file">


</div></td>
</tr>
<tr>
<td><div align="center"><font size="2" face="Arial, Helvetica, sans-serif">You
can send one file at the time.

If you want to send more the one file at the time you can make a zip
file and send it. </font></div></td>
</tr>
<tr>
<td> <div align="center">
<input type="submit" name="Submit" value="Send">
</div></td>
</tr>
</table>
<div align="center"></div>
</form>
</body>
</html>
===================================
file prova.php
<?
$file=$_POST['file'];
if($file)
echo $file;
else
echo "nop!!";
?>