support multipart/form-data (e.g. file uploads)

[chrisv2]

This patch enables file uploads by allowing multipart/form-data encoding in transport.

Usage example

(this took me a while to figure out, feel free to add this to the docs)

Server (Django)

from rest_framework import serializers, viewsets, parsers

# serializer with file field
class AttachmentSerializer(serializers.Serializer):
    file = serializers.FileField(write_only=True)

# viewset with upload method
class MyViewset(viewsets.ModelViewSet):
  # ...
  @detail_route(methods=['post'],
                serializer_class=AttachmentSerializer, 
                parser_classes=(parsers.MultiPartParser,))
  def add_attachment(self, request, *args, **kwargs):
      serializer = serializers.AttachmentSerializer(data=request.data)
      if serializer.is_valid():
          # serializer.validated_data['file'] is an instance of django.core.files.File
          print("uploaded file: %s" %  serializer.validated_data['file']

Client (JS)

const client = new coreapi.Client()
const schema = window.schema

// file must be either an instance of Blob (for content generated in JS) or 
// an instance of File (retrieved from an <input type="file"> or a dropEvent.FileList)

client.action(schema,
              ['mymodel', 'add_attachment'],
              {id: id, file: file})
      .then((result) => console.log(result))