jquery ile basitçe yapabilirsin, yaptığım örnek;

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<script type="text/javascript" src="http://code.jquery.com/jquery-1.4.min.js"></script>
<script type="text/javascript">
function inputEkle() {
    $('#inputalan').append('<div class="input"><input type="file" name="dosya[]" /></div>');
}
function inputSil() {
    $('.input:last').remove();
}
</script>
<style type="text/css">
body { font: normal 12px verdana; }
a { text-decoration: none; }
</style>
</head>
<body>
<form id="" action="<?php echo $_SERVER['SCRIPT_NAME']; ?>" method="post">
    <div id="inputalan">
        <div class="input"><input type="file" class="dosyainput" name="dosya[]" />&nbsp;<a href="javascript:inputEkle();">[+]</a>&nbsp;<a href="javascript:inputSil();">[-]</a></div>
    </div>
    <br />
    <input type="submit" name="upload" value="Yükle" />
</form>
</body>
</html>