Some software exports markdown files with .markdown or .mmd extensions. (I believe .mmd has been used for MultiMarkdown files in the past.)
Obsidian only seems to recognize .md files in a vault. Please consider recognizing the other two extensions, .markdown and .mmd, in addition to .md.
No big deal – can always rename files if needed – would rather not have to though.
I really wanted to take Obsidian for a spin, but yeah, it doesn’t recognize my hundreds of files with the .markdown extension as Markdown files. 
Is this on the roadmap?
I don’t know if it’s on the roadmap or not, but you could always programmatically rename these files!
On Linux, or with GNU utilities installed on a Mac, you would use:
while read f; do mv "$f" "${f%%.markdown}.md" ; done < <(find . -iname '*.markdown')
I’m sure there are some Windows wizzs here who could help you with that if necessary!
It’s simple to change file extensions in macOS in the Finder app. But that’s not what this thread is about.
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.