Mozilla Thunderbird and the p7m files attachments

p7m files are digitally signed documents and Mozilla Thunderbird manages those attachment with ease, displaying it in the e-mail body if positively recognized. Unfortunately for Italian users, maybe due to a not standard application of file format, when received a message with one or more file attachment of type application/pkcs7-mime (.p7m) is impossible to see the file and to save it.

It's a known issue (see bug 243833) and it's possible to fix installing the P7MONadd-on.