mboxbeat cycles through your mboxen and outputs JSON for each message, including attachments.
mboxbeat is meant to centralize your local mail spools on your linux servers without forwarding the emails to just another mailbox.
The intent is to send them to some form of noSQL database so they can be aggregated and queried, both ElasticSearch or MongoDB come to mind.
While mboxbeat can process attachments, I heavily recommend against processing them. This is not an email-forwarding service, it is meant to serve as a logging vessel for centralizing, aggregating and querying emails.
To that end, you are more interested in the text and metrics of than you are with downloading and saving attachments.