It is a separate table because we allow expenses to not have such an
attachment, although we allow only an attachment per expense, and i do
not want to have a bunch of nullable columns for that.
I decided to keep the files in the database, contrary to “conventional
wisdom” of storing files in the filesystem, because these attachments
are invoices and such documets that are an integral part of the expense
relation. In other words, losing these files would render the expense
(almost) useless. Thus, the ACID guarantees of the database are the
most appropriate place for them.
