Activities filed on Cases - custom file attachments lost on edit

If you have an activity filed on CiviCase, and that activity has a custom field that is a file upload, that data appears to be lost on any subsequent edit. The original file is still lurking in the file system somewhere, but is no longer shown on screen, accessible from the view option for the activity etc.

Steps to replicate on demo Drupal 7, CiviCRM 4.4.5:

• Create custom field set + field with file upload type, apply to a type of activity available for a case (I used "Follow Up")
• Create a case against any contact
• Create an activity on the case of the type referred to above, attach a file in the new custom field and save.
• Edit the case and save, without updating/editing the previously uploaded file.
• Attempt to edit the activity again - file is now gone

This appears to be related to version control for activities associated with cases as I could not replicate this with an activity that isn't on a case.