Balsa will use your GnomeCard address book, if you have one, to read and store contacts. It can also use LDAP to access a remote e-mail directory. This will allow you to see the most current filter code. This code is not in active development, but new filter tools are being worked on. On the other hand, the POP3 filtering backend based on procmail is functional.
Balsa does support attachments.
When I click on a jpg in Balsa it immediately displays the image for me in the preview pane. Any other quick graphic viewer will do. Easy as pie. In the process of autodetecting your mailboxes Balsa will recurse into the subdirectory and put the mailboxes in their own subdirectory. Balsa can read many mailbox formats natively.