Cannot see .rpmsg attachment to email on Mac

The US government sometimes sends emails with files encrypted as .rpmsg attachments. I just received another one from the Department of Energy for a project in which we are participating. It is important for me to view the attached files, but when I open the email I cannot have access to the attached files. This is important, but no one has ever given a fix in this thread (although this is a common issue). HELP!!! Lol!

[Re-Titled by Moderator]