Fix DownloadTargetHelper::determineDownloadTarget on Windows

filePathForCallback.AddExtension won't accept const char[]
with MSVC. It  needs to be converted with the appropriate function.

Change-Id: I95506de702eab4d7b61ddf051c7a122c793da35b
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>