Render API callback: Validates the maximum upload size field.
Ensures that a size has been entered and that it can be parsed by parse_size().
This function is assigned as an #element_validate callback in file_field_instance_settings_form().
function _file_generic_settings_max_filesize($element, &$form_state) {
if (!empty($element['#value']) && !is_numeric(parse_size($element['#value']))) {
form_error($element, t('The "!name" option must contain a valid value. You may either leave the text field empty or enter a string like "512" (bytes), "80 KB" (kilobytes) or "50 MB" (megabytes).', array(
'!name' => t($element['title']),
)));
}
}