Yeah, other apps exporting textbundles are coming with a .markdown file instead of .md. I thought it was a bug when I first tried to open the .markdown and it went over to TextEditor (default app). I changed the file name to .md and it opened in Obsidian, no problem for me.
The big problem here is a new user to Obsidian wouldn’t understand why their “markdown” .markdown file isn’t opening in an app that is designed for markdown.