Getting rid of the p7m file extension

Create a small shell script like this one:

#!/bin/bash
openssl pkcs7 -inform DER -in "$1" -print_certs -out "$1.pem"
openssl smime -verify -in "$1" -inform der -noverify -signer "$1.pem" -out "$1.pdf" 2>/dev/null
rm "$1.pem"
evince "$1.pdf"
rm "$1.pdf

1. Make it executable
2. Select "open with other application"
3. Choose the small script created above
4. Done.

Unfortunately, none of these answers worked for me. The attachment showed up in gmail's web interface as smime.p7m, and in Thunderbird (which I can now get rid of since it didn't help) as winmail.dat.

Quickly, as a list of commands, what worked for me was the following (assuming you downloaded the smime.p7m into ~/Downloads):

sudo apt install mpack tnef
cd ~/Downloads
munpack smime.p7m
tnef winmail.dat
ls -lt
rm mail.eml winmail.dat

To determine if this will even help you:

• just install mpack
• run munpack on the smime.p7m file
• A new file should appear (perhaps named winmail.dat). If this is the case, run:

file winmail.dat # or whatever the new filename

• if "winmail.dat: Transport Neutral Encapsulation Format" appears, install tnef and complete as listed above.
• if that completes successfully, you should now have your zip archive in the same directory.
• otherwise, this answer won't help:

sudo apt remove mpack tnef

Sources:

FIX Gmail smime.p7m and tnef open mail (with winmail.dat)

How to open winmail.dat files on Ubuntu and Debian Linux

p7m is an encryption format that is often used for email attachments.

I was also not able to find a Linux tool that is able to open that file type after a quick google research. However, there exist some tools for Windows (e.g. Cryptigo p7mViewer) or OSX.

What you could try if you have no access to a computer running Windows/OSX is to install the Windows emulator wine and try to install a Windows viewer in there. I can't tell you whether this would work, but it is worth a try, if that file is important for you.

Another possible solution could be to install Claws Mail, a mail program for Ubuntu, and this plugin. I am not sure whether that allows you to view those .p7m files with that mail program either, I just got this hint from @Toroidal in the chat.