使用django rest框架发送图像

Question

我想使用DRF发送图像以响应某个网址，为此我写了这些代码：

#Renderer class
class ImageRenderer(renderers.BaseRenderer):
    media_type = 'image/png'
    format = 'image'

    def render(self, data, media_type=None, renderer_context=None):
        return data  

#view class
class ShowImage(APIView):    
    renderer_classes = (ImageRenderer,)    
    def get(self, request, format=None):
        print ('format', format)
        if format == 'image':
            image_file = open('path_to_image', 'rb')
            response = HttpResponse(image_file, content_type='image/png')
            response['Content-Disposition'] = 'attachment; filename={}'.format('image_filename')

#urls.py
urlpatterns = format_suffix_patterns([
    url(r'image/?$', views.ShowImage.as_view())
])

但我的问题是输入format总是None，尽管request.accepted_media_type向我显示image/png

我使用httpie尝试了这些请求：

http -vj 127.0.0.1:8000/api/image Accept:image/png
http -vj 127.0.0.1:8000/api/image.image Accept:image/png

但我无法得到我期望的格式。使用DRF时我的错误是什么？