Now that Kleopatra has support to parse MIME data we can add a file extension other then .txt to the GpgOL MIME Structure which will then be opened by Kleopatra.
What is this GpgOL_MIME_structure.txt? It is the data construct used by GpgOL to create PGP/MIME and S/MIME Mails. Basically a mail without a body and some special MAPI flags will use the first attachment as the MIME structure when it leaves Exchange and is transferred over standard protocols.
If it is not converted then GpgOL handles this and uses that attachment as "the mime structure". But if you look at it with a client that has no GpgOL, like the outlook.com web interface or another outlook you will see that GpgOL_MIME_structure.txt thing. The filename is completely arbitrary. I used .txt so that users could look at it in their text editor and see what is in there, maybe even paste the relevant parts into the Notepad.
Here is an example:
This can also be S/MIME or could be a PGP/MIME signed mail. Basically the "MIME Body" without the headers.
I am currently thinking of using gpgol_mime_structure_.mim
And then add support for Kleopatra to open ".mim" files.