filefield validation in .module file

Hi,
I'm trying to mimic the #required validation for filefield inside my module.module file. I've tried the following, without any success:

if ($node->field_fattachment[0]['filepath'] == NULL) {
        form_set_error('File Attachment', t('You did not upload your file, please do so in order to submit your work'));
      }

or

if ($node->field_fattachment[0][value] == NULL) {
        form_set_error('File Attachment', t('You did not upload your file, please do so in order to submit your work'));
      }

or again

if (is_null($node->field_fattachment[0][value])) {
        form_set_error('File Attachment', t('You did not upload your file, please do so in order to submit your work'));
      }

I know it's possible to validate cck fields inside the .module file, because I am able to do it for checkboxes and text fields, but not with the filefield, it's like I can't find what to check, somehow... Any ideas?

THANKS!