Email Signature showing up as winmail.dat

Are we no longer able to add specialized email signatures to outgoing email in the Outlook desktop app? I'm on a Mac. I've tried multiple ways and when I add an image to the signature, it shows up as a winmail.dat attachment on the recipient end.

I don't see a way to change the email format to HTML and read that's no longer an option for the Outlook app. If that's the case, what are my options for creating a modern email signature?