S/MIME Signed (not encrypted) mails with attachment on an exchange account are missing the attachments. Instead it only shows an attachment called "smime.p7m". This didn't happen with the Gmail app. If a signed mail is received on a Gmail account (via IMAP), the attachments show correctly (also that the signature is valid).
In the attachments you will find a screenshot of a signed mail with attachment and embedded image. Additionally a screenshot of an unsigned mail with similar content is included. Finally you will find the corresponding "smime.p7m" file (just a multipart message in MIME format).
My Aquamail version is 1.32.1-072.