attachment: fixup r10553, in which the second `sum(attachment.size for attachment in attachments)` consumed the `attachments` iterator, resulting in an empty .zip.
attachment: fixup r10553, in which the second `sum(attachment.size for attachment in attachments)` consumed the `attachments` iterator, resulting in an empty .zip.

Reuse code from `attachment_data()` which was split into a `viewable_attachments()` method.

git-svn-id: f68c6b3b1dcd5d00a2560c384475aaef3bc99487@10588 af82e41b-90c4-0310-8c96-b1721e28e2e2

exocad/exotrac,exocad/exotrac,exocad/exotrac,exocad/exotrac