The whole idea is to allow uploading to our servers from their interface, not redirecting to another domain, regardless of auth method.

Another solution would be combining multiples. That could be options of either submitting SteemConnect access token or using a one-time upload_token. The API may accept either inputs.

Right now an investigation is being carried out to find out how our users are logging into our uploader. Let's see how it turns out here after 7 days.