boto3.s3.transfer set Metadata incorrectly

http://boto3.readthedocs.io/en/latest/_modules/boto3/s3/transfer.html says:

Setting metadata by documentation: transfer.upload_file(‘/tmp/myfile’, ‘bucket’, ‘key’, extra_args={‘Metadata’: {‘a’: ‘b’, ‘c’: ‘d’}})

If I upload a file, e.g. index.html with the following command:

from boto3.s3.transfer import S3Transfer
awsclient = boto3.client('s3', 'eu-west-1')
transfer = S3Transfer(awsclient)

transfer.upload_file("index.html",OUTBUCKET,"index.html",extra_args={'Metadata': {'Content-Type': 'text/html'}})

It sets up the S3 object metadata to: x-amz-meta-content-type: text/html

Instead of the expected: Content-Type: text/html

The main problem e.g.: uploading the index.html file with boto3 to an S3 bucket with static website hosting option, results the WEB browsers downloads it instead opens.

[grahamlyons]

@stealthycoin is right - calling the API with an invalid key shows that Metadata is a different key from ContentType in the API:

ValueError: Invalid extra_args key 'metadata', must be one of: ACL, CacheControl, 
ContentDisposition, ContentEncoding, ContentLanguage, ContentType, Expires, 
GrantFullControl, GrantRead, GrantReadACP, GrantWriteACP, Metadata, RequestPayer, 
ServerSideEncryption, StorageClass, SSECustomerAlgorithm, SSECustomerKey, 
SSECustomerKeyMD5, SSEKMSKeyId, WebsiteRedirectLocation

[mikeatlas]

Agreed, it is confusing. The AWS Web Console treats Content-Type as a “Metadata” property to set on the object, while the API gives Content-Type a primary parameter as well as a Metadata parameter, which, if you use the latter to try to set Content-Type, you get x-amz-meta-content-type key added.