Apple Mail not downloading attachments
I found this post to be very interesting: 

http://osxdaily.com/2016/12/17/fix-mail-not-showing-attachments-mac/


Backup your computer 
Rebuild your email mailbox(es) (this may take a long time to run, be sure your connected to internet because it's going to reload all the mails) 
In your "Account Information", select Download Attachement that matters to you (all)
Restart Mail.


Hope that helps