remove unused imports
don't delete attachments when you upload a multimedia file
update comments and add logging message
remove nonsensical replace_attachment param from attach_data

it was no longer doing anything.
remove unused replace_existing param and form option
remove now redundant template
remove confusing current_attachments property, replace with attachment_id
Merge pull request #1443 from dimagi/multimedia-hotfixes

Multimedia hotfixes for bad attachment errors