hook_preprocess_field() $variables crashing while printing
Use Devel with vardumper and dump only the $element instead of the whole $variables something like:

function myfi_preprocess_field(&amp;$variables) {
  $element = $variables['element'];
  if ($element['#field_name'] == 'field_download') {
    // Dump only the element.
    dpm($element);
  }
}