code: def init_multipart_upload(self, bucket, object_name, content_type=None,
                                amz_headers={}, metadata={}):
          
          objectname_plus = '%s?uploads' % object_name
          details = self._details(
              method=b"POST",
              url_context=self._url_context(bucket=bucket, object_name=objectname_plus),
              headers=self._headers(content_type),
              metadata=metadata,
              amz_headers=amz_headers,
          )
          d = self._submit(self._query_factory(details))
          d.addCallback(
              lambda (response, body): MultipartInitiationResponse.from_xml(body)
          )
          return d
docs: Initiate a multipart upload to a bucket.
  
          @param bucket: The name of the bucket
          @param object_name: The object name
          @param content_type: The Content-Type for the object
          @param metadata: C{dict} containing additional metadata
          @param amz_headers: A C{dict} used to build C{x-amz-*} headers.
          @return: C{str} upload_id