hook_imagefield_file not passing $file as reference?

Hiho

I'm trying to edit the title and alt values of $file before it's passed to the content.module to be inserted into the DB.
So I implemented the hook module_invoke_all('imagefield_file', 'save', $file) like this:

function mymodule_imagefield_file($op, &$file) {
  switch ($op) {
    case 'save':
      $file['alt'] = 'test 123';
      $file['title'] = 'test 123';
      break; 
  }
}

Any idea why this isn't changing the alt and title of the image to 'test 123'?

If I hack it directly into imagefield_file_insert() in imagefield.module after line 260 it works,
so there must be some problem with the passing by reference of $file (or me not getting php references). Thanks!