Can you more fully explain the issue you are encountering? You've filed this bug against Test Studio Ultimate, but it seems
like you're only talking about Fiddler and Visual Studio. Is Telerik Test Studio involved at all in your scenario?
Either way, please provide the full set of steps needed to reproduce the problem you're reporting.
Fiddler's "not fully supported" message concerns the display of the uploaded data in the WebForms Inspector only, and has nothing to do with the import features of TestStudio or Visual Studio.
A HTTP/415 error suggests that the server didn't like the client's Content-Type request header.