@baris Confirmed it is a bug. I think this was one of the first bugs I found so I wasn't sure if it was a misconfiguration or not.
Error Imgur & Extended Markdown
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To fix the invalid-files issue see the discussion in this thread https://community.nodebb.org/topic/15075/nodebb-v1-15-0-rc-0-release-candidate-published/7.
Specifically you need to update the multiparty dependency.
You can ignore the second it is just a warning, that plugin will need a update to use the slugify module in the future.