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)

fromrest_frameworkimportserializers, viewsets, parsers# serializer with file fieldclassAttachmentSerializer(serializers.Serializer):
file=serializers.FileField(write_only=True)
# viewset with upload methodclassMyViewset(viewsets.ModelViewSet):
# ...@detail_route(methods=['post'],serializer_class=AttachmentSerializer, parser_classes=(parsers.MultiPartParser,))defadd_attachment(self, request, *args, **kwargs):
serializer=serializers.AttachmentSerializer(data=request.data)
ifserializer.is_valid():
# serializer.validated_data['file'] is an instance of django.core.files.Fileprint("uploaded file: %s"%serializer.validated_data['file']

Client (JS)

constclient=newcoreapi.Client()constschema=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))