This method prevents excessive calls to the S3 API, creating a bucket
only if the store method fails with a NoSuchBucket error instead of
asking for the bucket before each request.

Leverage browser caching and proxy caching by setting far future Expires
headers and changing filenames when file contents change. This can make
your web app faster for users and also reduce your bandwidth costs.

By adding the column :avatar_fingerprint to our db table and including
:fingerprint in the attachment filename, we ensure the filename will
change whenever the file contents do.

  has_attached_file :avatar,
    :styles => { :medium => "300x300>", :thumb => "100x100>" },
    :path => "users/:id/:attachment/:fingerprint-:style.:extension",
    :storage => :s3,
    :s3_headers => {'Expires' => 1.year.from_now.httpdate},
    :s3_credentials => "#{RAILS_ROOT}/config/s3.yml",
    :include_updated_timestamp => false

This enables us to set far future expire headers so that browsers
don't need to check for a newer version. If a change does occur,
say because a user uploads a new avatar, the new filename will
be rendered in your html and the cached version will be ignored.

The example above will set Expires headers in S3. If you're using
local storage you can configure your webserver to do something similar.

We disable the timestamped query string because some proxies refuse
to cache items with query strings.

For more info on optimizing for caching:

http://code.google.com/speed/page-speed/docs/caching.html

Based on a commit by Henrik N <henrik@nyh.se>

Rails.root returns a Pathname and you can now declare your attachment
without having to call #to_s first:

has_attached_file :avatar,
                  :storage => :s3,
                  :s3_credentials => Rails.root.join('s3.yml')
(cherry picked from commit 65d7600b7246cb3bd8f10805fb618e5839694318)

Added a rake task for testing all versions. Changed gem statements to use ~> instead of hard versions.

These callbacks are different from pre-rails 3. They do NOT cancel the
after_ if the before_ returns false. They will cancell inner callbacks,
however, but that's it. This is a change in functionality.

Determine content_type with file command's --mime-type when content_type is not found in the default cases.
(cherry picked from commit 2ca52df1)

styles encapsulated, to delay calling of procs until context is useful and to allow per-style processor lists