Please note that Simple Fields is not actively developed anymore.
Simple Fields is a WordPress plugin that adds visual Custom Fields, hence making them more usably for end users (aka customers).
This version here at GitHub is the development version. Please see http://wordpress.org/extend/plugins/simple-fields/ for the latest and stable version.
Works great for attachments
Mix fields any way you want to
There are many built in custom field types in Simple Fields:
Use the GUI in WordPress or use PHP directly in your functions.php like this: https://gist.github.com/3851387
<?php
$textfield_output = simple_fields_value("my_text_field");
$file_attachment = simple_fields_value("my_file_field");
?>