Rather than trading performance to avoid the landmine of the
Transaction->Attachments cache, we'll just built our own single element
attachment collection.

Calling RowsPerPage on the value returned from Transaction->Attachments
means that any future code that calls Attachments on this Transaction
object will only ever return a single attachment.  This fails
predictably when Transaction->ContentObj wants to iterate through all
your attachments but only gets 1.
(cherry picked from commit 277ee932)