In the composer app, there is no way to retrieve the attachments of the mail you are writing.
I would be nice to have the same functionality as in read app, where you get the attachments using Office.context.mailbox.item.attachment
Thanks for our feedback! Although we appreciate your time and effort to give us this feedback, due to performance reasons, it’s not something we’re planning to do right now. We do not want to stream attachment bytes from server into client and into iframe of add-in. Instead, you can save the message, listen for notifications on send folder and then sync down the attachments using REST APIs.
Office Extensibility Team