Sorry to bring back an old topic, but I have the same problem with mails from iOS Mail.
If you send a photo by mail using an iCloud account, without adding a text (just the normal "sent with my iPhone" footer), it is received as a mail with an empty body, and two attachments (ATT0002.txt and the image).
I have inspected the eml, and it is a multipart/mixed with two text/plain parts (one empty, one with the footer, both without content disposition) and the attachment (with content disposition set to inline). I know this is probably not the way it's meant to be, but using MailKit for example, I can parse the mail just fine (it returns two body parts and one attachment).
The same happened to me yesterday with an HTML mail, which came in empty with a ATT00001.html and a pdf attachment. That's why I remembered the problems I had with iCloud/iOS mail and tested it again.
Is there any way to fix this (or maybe even a setting, though I haven't found any?), other than looking for ATT...txt/html attachments and parsing them manually? Thanks!