Favicon 설정: "CDN에서 올바르게 작동하려면 png여야 합니다"

Any CDNs (content delivery networks) you may have configured for hosting files when setting up your site, for example: Configure an S3 compatible object storage provider for uploads

If you aren’t using a CDN like this, then you can likely disregard the message.

The short answer is that Discourse expects a PNG on the backend.

The long answer seems to be related to this comment in the related code, though I’m not familiar with these technical specifics: