Forwarded emails with attachments showing as MIME gibberish
It seems like it is a bug that doesn't have a fix. That's what I understand from the 4/17/17 posted here: https://issuetracker.google.com/issues/36907768

Also, no idea if this will work but it might be worth a try: https://forum.xda-developers.com/showthread.php?t=2079469