Thank you for collaborating to do the work of Microsoft engineers who caused this problem in the first place.
The three files do not open nor show properly as attachments. Only after I save them I can do Get Info. They are saved as Mail Attachment.ics and Mail Attachment.eml (twice). But in the mail message they appear broken.
When I see the saved documents they get the icon of the associated apps: Calendar and Mail. Nevertheless, I try what you suggest and associate Calendar with the Mail Attachment.ics and then I get a warning:
"macOS cannot verify the developer of “Mail Attachment”. Are you sure you want to open it?"
Also, when I just click on this file I get "“Mail Attachment” cannot be opened because it is from an unidentified developer."
When I open the attachment from Mail it does open Calendar and it becomes an appointment in Calendar, as intended by the creators of Microsoft Outlook/Office, whatever the software is called that my correspondents are forced to use by the organisation they are in.
Interestingly, when I look at the raw source of the message I get this:
--_000_5519dbba138244b3a22fadc87d7b742eamsterdamnl_
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
<html xmlns:v=3D"urn:schemas-microsoft-com:vml" xmlns:o=3D"urn:schemas-micr=
osoft-com:office:office" xmlns:w=3D"urn:schemas-microsoft-com:office:word" =
xmlns:m=3D"http://schemas.microsoft.com/office/2004/12/omml" xmlns=3D"http:=
//www.w3.org/TR/REC-html40">
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Diso-8859-=
1">
<meta name=3D"Generator" content=3D"Microsoft Word 15 (filtered medium)">
The source reads as one long HTML document with some binary parts included, each of the three section preceded with a heading. Something goes wrong there I think. My guess is that Microsoft has a 'better' implementation of some standard for rich mail messages, but it is not adopted by the rest of the world. And therefore it is not a standard, but some Microsoft peculiarity/poison.
