<!DOCTYPE html>
<html>
<head>
<meta charset='utf-8'>
<script type="text/javascript">
function addField() {
document.getElementById("file2upload").insertAdjacentHTML('afterend','<input type="file" multiple="multiple" name="upload_file" size="50" value=""><br>');
}
</script>
<title>Title</title>
</head>
<body>
<form method="post" action="/upload.cgi" enctype="multipart/form-data">
Загрузить файл: <div id="file2upload"><input type="file" multiple="multiple" name="upload_file" size="50" value=""></div>
<br><a href="#" onClick="addField(); return false;">Добавить поля</a>
<input type="submit" name="abc" value="Загрузить"></form>
</body>
</html>