Compares two attachments such that the attachments may be sorted from newest to oldest:
If o1.getCreated() < o2.getCreated(), returns a positive integer.
If o1.getCreated() == o2.getCreated(), returns 0.
If o1.getCreated() > o2.getCreated(), returns a negative integer.
If the attachment itself or its create date is null (if this is possible), it will be treated as older
than valid attachments and attachments with valid create dates.