Is there a utility that can open a .RPMSG secured email file?



Last week I was sent an email message containing a 1099-R tax form so I could file income taxes on behalf of my 91 year old mother that resides in another state. The 1099 was attached to to an email i opened in Outlook 2016 but the extension on the attachment was .RPMSG and my Windows 10 Pro/Outlook 2016 system indicated that it could not open a message with that file format. I was prompted to identify an application that could but I was unaware of any app on my system that would. I searched the internet and it said that to open a message with that format I needed to have Office 365 but I'm not willing to upgrade to something that I don't need. I eventually get her pension provider to send me the file as a PDF but it was a hassle trying to reach a "real person" and I had to jump through a bunch of hoops just to get them to cooperate. Now that the dust is clear I thought I would ask someone on this forum if there are other options/utilities available hat would allow me to open a message in that file format short of installing Office 365. Any help or advice would be appreciated.