Change Content-Disposition of existing S3 object

Edited for clarity:

Content-Disposition must be set explicitly and not included as x-amz-meta-Content-Disposition. All metadata header names must start with "x-amz-meta-" and be all lowercase.

Thanks to @Eyal for clarifying.

Original:

>SOLVED:
>
>The Doco at http://docs.amazonwebservices.com/AmazonS3/latest/dev/index.html?RESTAuthentication.html
>
>seems to be wrong it says:
>
>Notice how the 'x-amz-' headers are sorted, white-space trimmed, converted tolowercase, and multiple headers with the same name have been joined using a comma toseparate values.
>
>Note how only the Content-Type and Content-MD5HTTPentity headers appear in the StringToSign .The otherContent-* entity headers do not.
However Content-Disposition must be set specifically and not included as : x-amz-meta-Content-Disposition
>
>It now works fine.

here: this uses the cli to set the content-disposition header on all files in a path inside a bucket (and also sets them as public):

aws s3 ls s3://mybucket/brand_img/ios/|awk {'print $4'} > objects.txt

 while read line; do aws s3api copy-object --bucket mybucket  \
--copy-source /mybucket/brand_img/ios/$line --key brand_img/ios/$line \
--metadata-directive REPLACE --metadata Content-Disposition=$line --acl public-read; done < objects.txt