在vaadin中上载图像时发生NoOutputStreamException

正在尝试仅上载图像。问题是我不知道如何处理这种情况。

    @Override
    public OutputStream receiveUpload(String filename, String mimeType)  {
        FileOutputStream fos = null;

        if(mimeType.startsWith("image")) {
            picture = new Picture();
            picture.filename = filename;
            picture.mimeType = mimeType;

            picture = HibernateUtils.save(picture);
            product.pictures.add(picture);
            File dirs = new File(IMAGE_LOCATION);
            File file = new File(IMAGE_LOCATION + picture.id);
            if (!dirs.exists()) {
                dirs.mkdirs();
            }
            try {
                fos = new FileOutputStream(file);
            } catch (FileNotFoundException e) {
                throw new RuntimeException(e);
            }

        } else {
            Notification.show("", Notification.Type.ERROR_MESSAGE);
        }
       return fos;
    }

问题是 fos 变量将为 null 如果我尝试上载不同类型的文件 image 。问题是,我该如何处理?