managed_file cleans form fields

Thanks for your answer.
It seems a bug to me as I loose form data, and I don't think it should happen. I don't know if this is a Drupal issue due to the way form data are handled, or a Ubercart issue due to the values passed to hook_add_to_cart_data, so I posted it here under Ubercart project.

Here's the code, hoping someone finds it useful.

<?
/** hook_form_alter
*/
function blizzart_form_alter(&$form, &$form_state, &$form_id) {
  if (isset($form['#id']) && (strpos($form_id,'uc_product_add_to_cart_form') === 0)) {
    // ...
	$form['upload-img'] = array(
	  '#type' => 'fieldset',
	  '#title' => t('Upload Image'),
	  '#weight' => -3,
	  '#collapsible' => TRUE,
	  '#collapsed' => TRUE,
	); 
	## Adds the image upload element to the form
	$form['upload-img']['image'] = array(
//
// TO HIT ERROR REPLACE NEXT LINE WITH: '#type' => 'managed_file',
//
		'#type' => 'file', // managed_file is buggy and clears the other form fields.
		'#title' => t('Custom Label'),
		'#weight' => '0',
	);
	
	// ... 

	$form['template-img'] = array(
	  '#type' => 'fieldset',
	  '#title' => t('Choose from Template'),
	  '#weight' => -20,
	  '#collapsible' => TRUE,
	  '#collapsed' => FALSE,
	);
	$form['template-img']['galleria'] = array(
		'#markup' => '<div class="jMyCarousel">'.$img_gallery.'</div>',
		'#weight' => -3,
	);
	$form['template-img']['img-scelta'] = array(
		'#markup' => '<div id="img-scelta-'.$gallery_path.'" class="bcViewDiv bcViewDiv'.$gallery_path.'" ><img class="bcView bcView'.
			$gallery_path.'" src="'.base_path().drupal_get_path('module', 'blizzart').'/'.$gallery_path.'/gallery'.
			'/default.jpg"></div>',
		'#weight' => -1,
	);
	
	// hidden field
	// --> this looses its value using managed_file
	$form['template-img']['gallery_path'] = array(
		'#type' => 'hidden',
		'#id' => 'gallery_path',
		'#value' => $gallery_path,
		'#weight' => -2,
	);
}

/** Ubercart hook
*/
function blizzart_uc_add_to_cart_data($form_values){
	// ..
	
	// custom function, see below
	$newvars = link_order_to_extradata($form_values);
	return $newvars;
}

/** Saves into the DB additional info. It calls another custom function based on the chosen gallery name
*/
function link_order_to_extradata($form_values){
	// ...

	// here hits an error. $form_values['gallery_path'] is found for no file uploaded into $form['upload-img']['image'],
	// while it gives an index array error if a file has been uploaded. 
	if (!is_null($form_values['gallery_path'])){ 
		// ...
	}
}