I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[x] Bug report  
[ ] Performance issue
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request
[ ] Other... Please describe:

RtReleaseAssets upload use a static uri uri: "https://uploads.github.com"
this.request.uri().set(URI.create("https://uploads.github.com"))

It should leverage the upload_url in the repository github response.