Can't upload file using test application

I am trying to use the test C# application included with the SDK. I can upload a text string and retrieve it just fine, but I cannot upload a file. I get the following error:

There was a mismatch between the signature in the request and the signature computed by the server.

The same Token and Shared Secret work for saving text, though. Is there a different kind of Token that I need to use for uploading a file?