A connection error usually means there is a server side error, can you check the logs/output.log when uploading an image in the composer.
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.