For example here: http://musicbrainz.org/release/4cee7e57-d829-4fa8-be24-7fd104755ee2/add-cover-art
After uploading the attached image as a back cover to mbid-4cee7e57-d829-4fa8-be24-7fd104755ee2.s3.us.archive.org (in a frame of the page) I receive HTTP status code 503 and this response:
------
<?xml version='1.0' encoding='UTF-8'?>
<Error><Code>SlowDown</Code><Message>Please reduce your request rate.</Message><Resource /><RequestId>7ef5763d-84e1-42c7-8b5b-56db67f6dd43</RequestId></Error>
------

Below are the POST data sent to http://mbid-4cee7e57-d829-4fa8-be24-7fd104755ee2.s3.us.archive.org/
I replaced data where I am not sure about sensitivity by ***.

-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="AWSAccessKeyId"

***
-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="acl"

public-read
-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="content-type"

image/jpeg
-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="key"

mbid-4cee7e57-d829-4fa8-be24-7fd104755ee2-2165559407.jpg
-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="policy"

***
-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="signature"

***
-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="success_action_redirect"

http://musicbrainz.org/release/4cee7e57-d829-4fa8-be24-7fd104755ee2/cover-art-uploaded?id=2165559407
-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="x-archive-auto-make-bucket"

1
-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="x-archive-meta-collection"

coverartarchive
-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="x-archive-meta-mediatype"

images
-----------------------------111339330020051427351893195441
Content-Disposition: form-data; name="file"; filename="8.557292r-rotcrop.jpg"
Content-Type: image/jpeg

....JFIF
...
----------------------------111339330020051427351893195441-