This validation was causing issue when there is no content_type column
in the database, as the content type of the attachment would be empty.
There should be no need to check attachment's validity because this
`#reprocess!` method is running internally on the server.

Fix #2078

Encountered an issue where the URI was returning header with
content-disposition where the filename value wasn't enclosed in the
double quotes. Turns out that this is a valid grammar according to
RFC6266. Also made the logic more robust to account for spaces and
uppercase letters.

e.g., the way google stores its user profile photos (from their oauth2 API)
https://lh3.googleusercontent.com/-aRHQV4YjnBM/AAAAAAAAAAI/AAAAAAAAAAo/ngSjy9-yR5c/photo.jpg

Since the support version is Ruby 2.0 or later, magic comment is unnecessary.

> The UTF-8 default encoding, which make many magic comments omissible

https://www.ruby-lang.org/en/news/2013/02/24/ruby-2-0-0-p0-is-released/

This cause to erease previous image when the id change to above
`999_999_999`, for example:

```
2.3.6 :010 > id
 => 1000602578
2.3.6 :011 > ("%09d".freeze % id).scan(/\d{3}/).join("/".freeze)
 => "100/060/257"
```

So that the `file_system` option renders correctly on the docs

Closes https://github.com/thoughtbot/paperclip/issues/2253

[ci skip]

* Added support for aws-sdk-s3 gem which is now preferred way to interact with s3.

Reference: https://github.com/aws/aws-sdk-ruby/blob/master/V3_UPGRADING_GUIDE.md#library-maintainer

* Drop support for aws-sdk gem

[ci skip]

There was a bug in the code where if you created a bucket the file would
get read but not rewind on the first pass through (when the bucket
didn't exist). So when we went into the retry logic the file would
already be read and zero bytes would get copied over from the file. This
fixes that by rewinding the file during the retry so it gets correctly
copied over the second time around.

Please see #2527 for details.

* Provide example for s3_host_name

Gives an example, since not everyone knows the format required.

* Shorten line per Hound comment

* Add migration_version variable to migration generator for rails 5 compatibility

* Update syntax to satisfy hound

* Ruby 2.4.0 compatibility (doen't use Fixnum anymore)

* Fix typo

* Use Integer instead of 0.class

[ci skip]

This is a year in the making, with a security patch motivating it.

[ci skip]

Remove the URI adapters. Few people use them by default and they can
allow insight into the internal networks of the server. If you want to
enable them, add (for example) `Paperclip.DataUriAdapter.register` to
your `config/initializers/paperclip.rb` file.

This is related to CVE-2017-0889.

Elsewhere fix CI: it's `s3.us-west-2` now, with a dot.