Which are the file_save_upload() validators? In the file_save_upload() function, what keys can $validators take? I cannot find answers anywhere.
You can see the validators in core at the end of this listing:

http://api.drupal.org/api/drupal/includes%21file.inc/group/file/7

the ones starting with file_validate_

Also there's an example of the array structure here: http://api.drupal.org/api/drupal/modules%21user%21user.module/function/user_validate_picture/7