If botocore is installed, Scrapy's S3FilesStore allows a limited number of headers to be applied when persisting files to S3. The headers are converted to their botocore s3 put_object method equivalent argument. The mapping however is incomplete.
If you don't have botocore installed you are able to persist a file with the storage class STANDARD_IA using the header x-amz-storage-class. With botocore, if you try to do the same thing, the type error Header "x-amz-storage-class" is not supported by botocore will be raised.