Hi @RbDev,
The test connection button in the System Console just makes sure the user can connect and check that it can see that the bucket exists, but it doesn’t check that you can actually put an object into the bucket or get one that’s already been uploaded. Can you confirm that the user that you’re using for Mattermost has s3:GetObject permissions?
If it does have have access to that, you could try turning on the FileSettings.AmazonS3Trace setting in Mattermost to see if there’s any more information logged on the server when you try to upload a file.
The only other thing I’d note is that ERR_CONNECTION_REFUSED usually means there’s an issue between your Mattermost server and the web browser, but seeing as the file was uploaded to S3, that doesn’t seem related.