FileUploadField should return FileUpload as its converted input

FileUploadField should return FileUpload as its converted input so the file can be validated.

Eg,
@Override
protected void convertInput() {
setConvertedInput(getFileUpload());
}

Apparently MultiFileUploadField handles this (I've checked, it appears to be in the convertValue method).

For more context see http://www.nabble.com/Best-way-of-validating-FileUploadField-td17662018.html