Webform - Validating File Upload with PHP How can I validate, via Webform module (using Webform file upload in Drupal 7) whether the file has been uploaded or not?

How can I handle and reference those files?

Seems that normal FAPI calls don't work (e. g. file_save_upload('file1') returns a non-true value if I have a file field).

I see that Webform file fields upload the file BEFORE form submission (i. e. clicking "upload to server" adjacent button). 

I don't know if that's the normal file-upload behavior or it's a specific behavior.

How can I handle the files in that way? I need to validate if at least one of those fields was uploaded (assume they are named foto1, foto2, foto3, foto4), and later I need to reference those files (e.g. getting a public link to them).
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.