In the Login screen, we would have an additional "File protocol:" value 'S3'.
Protocol Options for 'S3' (i.e. appearing to the right of the "File protocol:") would be 'HTTP' and 'HTTPS'.
Host name might want to be
s3.amazonaws.com - Puts bucket names as top-level directories
<bucketname>.s3.amazonaws.com - Provide access to a single named bucket
(I don't exactly know how S3 is structured, but I would guess that single-bucket names will cause requests to be sent directly to the servers of the S3 region (US / US-N.CA / Europe / AsiaPac) which the bucket is stored in, whereas using s3.amazonaws.com may cause requests to be routed via the US.)
Under protocol S3, the "User name:" label should be changed to "Access Key:" and the "Password:" label changed to "Secret Access Key".
"Port number:" should be defaulted to 80 or 443 based on the protocol options.
"Private key file" should be greyed-out.
I think "UTF-8 encoding for filenames" becomes irrelevant under S3, because the key names (effectively file names) are defined to be Unicode anyway (I think). (I think it will be URL percent escaping of UTF-8, but the point is that it is defined somewhere so there is no point having an option for it.)