Amazon’s S3 file hosting service is a scalable, easy place to store files for distribution. You can find out more about it at http://aws.amazon.com/s3 There are a few S3-specific options for has_attached_file:

• s3_credentials: Takes a path, a File, or a Hash. The path (or File) must point to a YAML file containing the access_key_id and secret_access_key that Amazon gives you. You can ‘environment-space’ this just like you do to your database.yml file, so different environments can use different accounts:

    development:
      access_key_id: 123...
      secret_access_key: 123...
    test:
      access_key_id: abc...
      secret_access_key: abc...
    production:
      access_key_id: 456...
      secret_access_key: 456...
  

  This is not required, however, and the file may simply look like this:

    access_key_id: 456...
    secret_access_key: 456...
  

  In which case, those access keys will be used in all environments. You can also put your bucket name in this file, instead of adding it to the code directly. This is useful when you want the same account but a different bucket for development versus production.

• s3_permissions: This is a String that should be one of the "canned" access policies that S3 provides (more information can be found here: http://docs.amazonwebservices.com/AmazonS3/2006-03-01/RESTAccessPolicy.html#RESTCannedAccessPolicies) The default for Paperclip is :public_read.
• s3_protocol: The protocol for the URLs generated to your S3 assets. Can be either ‘http’ or ‘https’. Defaults to ‘http’ when your :s3_permissions are :public_read (the default), and ‘https’ when your :s3_permissions are anything else.
• s3_headers: A hash of headers such as {‘Expires’ => 1.year.from_now.httpdate}
• bucket: This is the name of the S3 bucket that will store your files. Remember that the bucket must be unique across all of Amazon S3. If the bucket does not exist Paperclip will attempt to create it. The bucket name will not be interpolated. You can define the bucket as a Proc if you want to determine it’s name at runtime. Paperclip will call that Proc with attachment as the only argument.
• s3_host_alias: The fully-qualified domain name (FQDN) that is the alias to the S3 domain of your bucket. Used with the :s3_alias_url url interpolation. See the link in the url entry for more information about S3 domains and buckets.
• url: There are three options for the S3 url. You can choose to have the bucket’s name placed domain-style (bucket.s3.amazonaws.com) or path-style (s3.amazonaws.com/bucket). Lastly, you can specify a CNAME (which requires the CNAME to be specified as :s3_alias_url. You can read more about CNAMEs and S3 at http://docs.amazonwebservices.com/AmazonS3/latest/index.html?VirtualHosting.html Normally, this won’t matter in the slightest and you can leave the default (which is path-style, or :s3_path_url). But in some cases paths don’t work and you need to use the domain-style (:s3_domain_url). Anything else here will be treated like path-style. NOTE: If you use a CNAME for use with CloudFront, you can NOT specify https as your :s3_protocol; This is *not supported* by S3/CloudFront. Finally, when using the host alias, the :bucket parameter is ignored, as the hostname is used as the bucket name by S3.
• path: This is the key under the bucket in which the file will be stored. The URL will be constructed from the bucket and the path. This is what you will want to interpolate. Keys should be unique, like filenames, and despite the fact that S3 (strictly speaking) does not support directories, you can still use a / to separate parts of your file name.