Couldn't find MIME type application/octet-stream
From ArchWiki
"Couldn't find MIME type application/octet-stream" can be shown as an alert from KDE after it's updated. Can be solved when you create new mime type in KDE Control Center named 'octet-stream' in category 'application' and with the same text (octet-stream) in the description.
Check contents of octet-stream.desktop located in /home/userhomedir/.kde/share/mimelnk/application. Check to see that there is a line that reads Hidden=false. If Hidden=true you will still get the error message.