I have gone into Tools->Options->Content Viewers and associated .vcs and .ics extensions to pmical.exe. When I get an email with an attachment with one of these extensions, clicking Open opens it with pmical.exe.

I receive many Teams invitations that have an attachment called Calendar that has no extension. The description given to it by Pegasus is "Calendar invitation or information".

I have tried adding to "content viewers" an 'Attachment type' information that matches "Calendar invitation or information" . I also tried this and added .ics as the extension for Temporary File Handling. This did not work.

Is there a way to get pmical.exe to open these files?

Do you know whether these files will actually display with pmical?

I use five new mail filters that attempt to detect calendar attachments. One of them checks for .ics and .vcs extensions while the others are expression filters that check for the following content:

Content-Type: text/calendar
Content-Type: text/vcalendar
QkVHSU46VkNBTEVOREFS (Detects Base64 encoded "BEGIN:VCALENDAR" )
BEGIN:VCALENDAR

Perhaps searching the raw view for each of these could reveal something, although I don't know how it could then be utilized by the attachment Open button. Like you, I only have .ics and .vcs attachments working that way.

Yes they do.

I currently either forward using the "bounce/redirect" option to a gmail account and it displays all the information there.

I can also save the attachment as calendar.vcs and then use pmical.exe to open the saved calandar.vcs file and it shows the information.

Without the filename extension I think that best that can be hoped for is a new mail filter that detects its content-type (if unique) and activates pmical or one that detects the attachment filename (if always the same) and saves it to a file.

Its nice of Teams to not include a file extension on the "Calendar" file attached to the invite.

Outlook probably doesn't need it which is all they care about.