If you're talking about doing this in real-time then one possible approach…write an MsExchange listener

1. Before email delivery extract the attachment(s)
2. Use Nuxeo API to create Nuxeo document - with binaries and metadata
3. Use the UID (unique ID) from Nuxeo document creation to create a hyperlink to the Nuxeo document attachment (security will be a concern here - could use a unique generated token that get's stored with attachment & embedded in URL - either way you need a mechanism to secure assets stored in Nuxeo)
4. Save the hyperlink into the yet-to-be-delivered email
5. Delete the attachment from the yet-to-be-delivered email
6. Let the mail delivery complete

This is one possible approach - the best solution in your case depends on your requirements and goals. Speed of execution, security and single object storage are all important considerations in the solution. If an off-line solution is acceptable then a similar process can apply without the same need for speed of execution.