Be carefull with setting max_file_size via
<?php ini_get('upload_max_filesize'); ?>
ini_get might return values like "2M" which will result in non working uploads.
This was the "no no" in my case:
$form = '<input type="hidden" name="MAX_FILE_SIZE" value=".ini_get('upload_max_filesize')." />';
Files were uploaded to the server, but than there was not any upload information, not even an error message. $_FILES was completly empty.