I replied to this post, as it’s essentially the same issue, but in V8. (See, shows I searched before posting!)
Emails sent from an Android client (Samsung default mail program in my case) that include a Forwarded email that is HTML format (Base 64 encoding?), do not display correctly, but show as “Attachment.htm”, and a bunch of “Attachment.txt” files, attached to the originator’s Forwarding email message.
The headers are exactly as per the forum post linked to above.
I definitely think it has something to do with the way the email client interprets;
If the email is saved and opened in another email client (GMail, a Webmailer) it displays correctly. The problem exists with the client not being able to ‘handle’ the forwarded HTML Base 64 Encoded email message at the end of the originator’s message.
I think that’s about as much information as I can provide. It’s been more than 25 years since I supported SMTP and IMAP mail clients in an enterprise setting. We were all using Eudora! I’m a little out of my depth on all the new Android & iOS stuff.