Attachments fail on Activity with Custom Data

Attachments added to an Activity using the Attachment field list are not saved if the Activity has any custom data fields.

Discovered this bug on v3.3.5 and just confirmed on the demo site (v3.2) where someone added a blood test field (see activity http://drupal.demo.civicrm.org/civicrm/contact/view/activity?atype=45&action=view&reset=1&id=562&cid=111&context=activity which is not showing the attachment just added).