Downloads coming from S3 even with DISCOURSE_S3_CDN_URL set

I followed the guide at Using Object Storage for Uploads (S3 & Clones) to set up Backblaze B2 for upload storage and I’m seeing a strange problem.

If I create a thread and upload an image the img src is set to my S3_CDN_URL. If I upload a zip file the URL is at /uploads and redirect directly to my bucket instead of using the S3_CDN_URL. I do not have DISCOURSE_CDN_URL set - the guide indicate it doesn’t affect this issue.

Me relevant settings are:

  DISCOURSE_USE_S3: true
  DISCOURSE_S3_REGION: "us-west-000"
  DISCOURSE_S3_INSTALL_CORS_RULE: false
  DISCOURSE_S3_CONFIGURE_TOMBSTONE_POLICY: false
  DISCOURSE_S3_ENDPOINT: "https://s3.us-west-000.backblazeb2.com"
  DISCOURSE_S3_ACCESS_KEY_ID: "ID"
  DISCOURSE_S3_SECRET_ACCESS_KEY: "KEY"
  DISCOURSE_S3_CDN_URL: "https://devforum-b2-cdn.freetls.fastly.net"
  DISCOURSE_S3_BUCKET: "bucket-name"
  DISCOURSE_S3_BACKUP_BUCKET: "backup-bucket/devforum"
  DISCOURSE_BACKUP_LOCATION: s3

Why would zip files be handled differently than images?