The value in Binding.StringFormat is
not passed in to the Convert (or ConvertBack) method of a value converter.
When a binding has both a converter and a StringFormat, the source value is first sent through the converter:
value = converter.Convert(value, targetPropertyType, binding.ConverterParameter,
Then if the target property expects a string, the result is passed through String.Format:
value = String.Format(culture, binding.StringFormat, value)
If you want to pass some kind of format string into a converter, use the ConverterParameter.