When you upload an artifact (jar + json), the UI doesn't seem to make a request to the backend (checked in developer tools). It just responds with "UploadFailed: 400".
Also, it seems like the user experience can be improved in this modal since there is very little feedback.
e.g. Initially the cloud is grey, you upload a file using the browse button, and it turns green. However, the browse link is still active, and you can click on it. When you upload again though, there's no color change or any other feedback.
It may have been exaggerated because of the failure to send a request though.