I installed 3.1.0RC2 at 11.07.2014.

From that date it creates files like:
att_1405265528_581f0dd8a203720d.tar
att_1405268040_b25d6ea3a6eba27e.zip
att_1405330119_7401910750b93f75.tar.bz2
att_1405361890_d4ad3cf92dda8418.tar.gz
in the "store" directory.

Up to now: 2,9 GiB of these files.

Inside these archives, i have some or many of the board attachments.

Reproduce:

• go to a topic with attachments
• press --> tools --> download all attachments
• the packed-files will be created in "store"