Webform - Validating File Upload with PHP
Found it -.-''.

$form_state['values']['submitted']['foto1'] returns integer 0, which is a "no file" id, instead of another value (webform for Drupal 7 currently returns a numeric string as id). File extension validation and other rules have been executed beforehand, and finally the obtained id can be loaded with webform_get_file($form_state['values']['submitted']['foto1']), obtaining a managed_file entity (actually, as a stdClass instance).

This is the result of DPM-ing the four fields using dev tools in Drupal 7, where foto4 is not filled. The four entries correspond to calls to webform_get_file($form_state['values']['submitted']['foto1']) to foto4-corresponding field path.