If you're reading this, the EWS Managed API 2.2 is available for download. This version doesn't add much to the 2.1 version, but if you're working with attachments in a mail app, these changes were made with your scenario in mind.
The 2.2 release adds an Attachment property to the GetAttachmentResponse class. This seemingly simple change means that now you can use the EWS Managed API to retrieve attachments using only the attachment ID, as is the case when a mail app sends attachment IDs to a remote service, as described in the How to: Get attachments of an Outlook item from an Exchange server article. Look for that article to be updated with sample code using the EWS Managed API in the near future.
Download the 2.2 version here.