Hi Koen,
The MIME type for .msg files should be application/vnd.ms-outlook. But I’ve read that the browser is not returning that MIME type for .msg files.
Herewith a link to a known workaround: https://stackoverflow.com/questions/55687631/which-mime-type-can-i-use-for-msg-file-using-file-object
Cheers,
Jeffrey