Currently, if you attach during compose, you can only see the attachments listed during the review screen, and to actually check the contents of the attachments, you need to start a preview.

I see two possible improvements:

• the review page could be made more like the preview, except that of course all the review buttons would be there, but you could be shown the text/plain part of your email below this, and it should be possible to switch between this text/plain and attachments (when the attachment would be filtered like during view)
• the compose window itself could be similar: attachments at the bottom, and EDITOR swapped with filtered views of attachments if you cycle through