@countach that's nothing to be worried about for now. It's something that needs to be taken care of on the theme's end. Post an issue on the theme's repository.
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.