Hello Thomas Derenthal,
When you click OK you should see a second upload box which allows you to select the code file. Can you please let me know if you are uploading the master file.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items