attachment grid: default to filtering by content-disposition

attachment grid: default to filtering by content-disposition

Normally, attachments are given the 'content-disposition: attachment'
mime attribute, so we use that to filter the attachments.

But since we're dealing with spam & virus mails here, and that field
is client controlled, give the user a way to toggle the remaining parts
of the mail too.

Added the checkbox in the header part, but that made it necessary to
manually implement the expand/collapse toggle (since the tools are on
the wrong side of the default toggle)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>