No attachments with digitally signed emails?

[pittsmaster]

I have a certificate setup on my outlook. I digitally sign my outgoing emails. When I send emails with or without attachments to users with Androids or iPhones there is no issue. My boss uses a windows phone (nokia Lumia 928) and he gets the text of the email but he says he cannot see any attachments on his phone from me when I digitally sign my email. If i do not digitally sign my email then he does get them on his phone. He gets attachments when my email are signed or unsigned when he accesses his email from a computer, so his phone is not handling the digitally signed emails correctly. Again, iPhone and Android work with zero configuration for this. What does he need to do to his windows phone to receive digitally signed emails and also access the attachments? Thanks