Fixed stream upload error. Removed unused Upload_Source::process_type_stream()

`Upload_Source::process_type_stream()` was left behind when extracting `Upload_Util`, but it is used nowhere.

When specifying a path in the exception message, the absolute path on the server MUST not be revealed. That's what `Debug::path()` is for.

Also "(:t)" placeholder was left behind, but never used.