That appears to be the problem: application/octet-stream is MIME for “everything we download that isn’t simple text and we can’t figure out any other type for”.
I appear to also have this configuration, but I don’t suffer from the reported issue - maybe I’m doing something else, or I only open files for which there is well know file types (maybe there’s no good mime type for the .eml file you tried to open). One thing that you can try, is open System Settings and go to “File Associations”, then in the list, under “application”, find “octet-stream” and remove the “Open on connected device via KDE Connect” entry.