There was similar discussion in this thread: Twitter Ads API Integration With C# Application
Generally, the auth headers are very tricky to get wrong and having one be missing or have a single character different can cause it to fail. I am glad to take a look at the code if anyone has a crack at C# version of upload in Open Source project.
We are considering to open an SDK sub-forum here for coordination of SDK help within the community, so please also look out for that.