Post file to S3 from Elm/Elixir AWS4-HMAC-SHA256 - amazon-s3

Question

I am attempting to post to S3 from an Elm application. I have a backend service that generates the signature, policy, etc required to make the post.
I have read through the docs and many of the posts about how to post to S3 and the need to use AWS4-HMAC-SHA256, however, my code is still failing with the error.
Elixir Code to create signature
defp signature(signing_key, string_to_sign) do
hmac_sha256(signing_key, string_to_sign)
|> bytes_to_string
end
defp signing_key(secret_key, date, region) do
hmac_sha256("AWS4#{secret_key}", date)
|> hmac_sha256(region)
|> hmac_sha256(#service)
|> hmac_sha256(#aws_request)
end
def hmac_sha256(key, data) do
:crypto.hmac(:sha256, key, data)
end
def bytes_to_string(bytes) do
Base.encode16(bytes, case: :lower)
end
Elixir code to create signature
defp policy(key, mimetype, credential, date, expiration_window \\ 60) do
%{
expiration: now_plus(expiration_window),
conditions: [
%{bucket: bucket_name},
["starts-with", "$key", key],
%{acl: "public-read"},
%{success_action_status: "201"},
["starts-with", "$Content-Type", mimetype],
%{"x-amz-credential": credential},
%{"x-amz-algorithm": "AWS4-HMAC-SHA256"},
%{"x-amz-date": date}
]
}
|> Poison.encode!
|> Base.encode64
end
Elixir code to create credential
defp credential(date) do
credential(aws_config[:access_key_id], date)
end
defp credential(key, date) do
key <> "/" <> date <> "/" <> region() <> "/" <> #service <> "/" <> #aws_request
end
Elm Code that makes post
makeMultiPart : UploadSignatureModel -> File -> Http.Body
makeMultiPart uploadSignature file =
Http.multipartBody
[ Http.stringPart "key" uploadSignature.key
, Http.stringPart "acl" uploadSignature.acl
, Http.stringPart "success_action_status" "201"
, Http.stringPart "Content-Type" uploadSignature.content_type
, Http.stringPart "X-Amz-Credential" uploadSignature.credential
, Http.stringPart "X-Amz-Algorithm" "AWS4-HMAC-SHA256"
, Http.stringPart "Policy" uploadSignature.policy
, Http.stringPart "Signature" uploadSignature.signature
, Http.stringPart "AWSAccessKeyId" uploadSignature.aws_access_key_id
, Http.filePart "file" file
]
Obviously, I am missing something but for the life of me can't figure it out.
EDIT:
The error code the user gets is this:
<Error>
<Code>InvalidRequest</Code>
<Message>The authorization mechanism you have provided is not supported. Please use AWS4-HMAC-SHA256.</Message>
<RequestId>0B1FCA2C05E910B1</RequestId>
<HostId>7ydiqVEEPu22aN+o1BJhAQDQbDXBodChOfHv7986R8ItnhQ5hv0/iETzakTH8gLVljjqKr3lIUg=</HostId>
</Error>