Persist file attachments on file system rather than DB

So we stop cluttering the database with blob of datas

Change-Id: Ife2166b85a4534fac7d000e43a8a2e9a0c92a656
Merge commit 'refs/changes/01/901/2' of http://gerrit.tuleap.net/tuleap into HEAD

* http://gerrit.tuleap.net/tuleap:
  Persist file attachments on file system rather than DB
This is Tuleap 6.4.99.9