text: Title: Ability to choose file from UI
  Body: Hey @tfranzel,
  
  First of all, thanks for the great library! 
  
  I'd like to get a file picker to show up on the Swagger UI (I believe this is possible, but if not then let me know). My understanding was that I should be able to do that using the `FileUploadParser` and `extend_schema`. However, when I try to get it it just seems to think the field is a string field.
  
  Here's the relevant part of my code:
  ```
  class SamplesheetSerializer(Serializer):
      file = serializers.FileField()
  
  class SamplesheetView(APIView):
      parser_classes = [FileUploadParser]
  
      @extend_schema(request=SamplesheetSerializer)
      @action(detail=False, methods=['post'])
      def post(self, request: Request, filename: Optional[str] = None):
           ...
  ```
  
  And here's the Swagger it generates:
  <img width="1362" alt="Screen Shot 2021-07-12 at 1 57 49 PM" src="https://user-images.githubusercontent.com/7347808/125354855-276bdd00-e319-11eb-8007-c7da2ae931e3.png">
  
  Any help/hints would be greatly appreciated!