I’m not exactly sure what happened there, but from the screenshot it looks like they tried to open a message/rfc822 MIME typed file (.eml, though the Firefox dialog doesn’t specify the mime type directly - I thing this is what it means by saying that the file “is email message”). On my system both Visual Studio Code and KDE Connect are listed as file associations for message/rfc822 though none are the default.
It could just be incorrect default or an off-by-one index lookup, or something like that.