I am sorry – I can not reproduce the issue, as I am not allowed to create files with illegal characters on my computer.

The mail was sent to us from a client who works in a company that uses  Microsoft Outlook.
The guy who received it Works very closely with the client, and he also has a mailbox on the client's Company server.
He received the mail in both his mailbox in our Company and the client's Company, but he could only see the attachment in his 'client-mailbox'.
He has forwarded it to my Yahoo mailbox. I can see the attachment with the ugly name, but when I download it, it is called 'attachment.eml'.

I experienced the same as you when I sent it back to my company mail.
The attachment is deleted, but the text within the attachment is shown in the body field of the mail, just like a regular mail.
If I attach the file called attachment.eml and send it from Yahoo to my Company mail, I receive it as an attachment.

And I forgot - there is both a colon and a slash in the attachment name: 'FW: Svar til DNV på runway beam. File No. 170_0023 / 263421..eml' – so it is double illegal.

I am not sure what you mean by locale – we are situated in Denmark, so our regional settings are Danish if that is what you mean.

You are right that nothing is missing, but it would be nice if it was stated somewhere that eml attachments with illegal characters were treated as Notes rich text and inserted directly into the mails.
I am still not sure if it is a bug or this is how it is meant to be.
Regards, Karen