A ver, fijate si te sirve:
Código:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>test</title>
<script>
function control(f){
var ext=['gif','jpg','jpeg','png'];
var v=f.value.split('.').pop().toLowerCase();
for(var i=0,n;n=ext[i];i++){
if(n.toLowerCase()==v)
return
}
var t=f.cloneNode(true);
t.value='';
f.parentNode.replaceChild(t,f);
alert('extensión no válida');
}
</script>
</head>
<body>
<form action="" method="post" enctype="multipart/form-data" name="form1" id="form1">
<input type="file" name="file" onchange="control(this)" /><br />
<input type="file" name="file" onchange="control(this)" /><br />
<input type="file" name="file" onchange="control(this)" />
</form>
</body>
</html>