Uploaded files with required file fields not detected during validation

When a form contains a file filed that is marked as required, the _form_validate() function always rises the error 'field is required' even if the field was not empty. I found that the value of $elements['#value'] for file fields is always empty during form validation in _form_validate(). If this is normal behavior, than a special test for file fields is needed. One solution could be to replace:

if ($elements['#required'] && empty($elements['#value']) && $elements['#value'] !== '0')

with

if ($elements['#required'] && (($elements['#type'] == 'file' && !file_check_upload($elements['#title']) || ($elements['#type'] != 'file' && empty($elements['#value']) && $elements['#value'] !== '0'))))

But if the element should anyway have value, than the error is somewhere else.