As described in #2118, `OpenURI::Meta#content_type` will return
`application/octet-stream` when no `content-type` header is set. Using
the original `meta` fixes this issue.

While using the Paperclip gem, we noticed during some ad-hoc testing
that if you do not supply an extension when uploading a file, Paperclip
effectively skipped it's spoofing check, which allowed potentially
dangerous files to slip through into your application.

This addresses that by moving the checks around a little bit and only
testing against the extension when there is one.

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"
```

"key" is a reserved keyword in MariaDB, so the SQL statement fails.
Surrounding the keyword in backticks fixes the error.

* Add deprecation notice to README

Addresses new projects, existing projects, issues, and PRs.

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

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

Follow #2481

2254 coverage (#2395)

* f(x): default to Paperclip::ContentTypeDetector

* fix: failures

* spec: cover consecutive periods

* fix: hound

* fix: hound

- open-uri is a security concern, so let's not recommend it. Separate
one-liners based on whether they're doing local or remote storage.
- Mention the performance impact of the separate table. While migrating
they will want to consider each use case for n+1 loads.

Thanks, Derek!

A start at an introductory migration path for moving from Paperclip to
[ActiveStorage].

ActiveStorage is coming in Rails 5.2 (April, perhaps) and we want to
help people move from Paperclip to it. Having a file upload strategy
baked into Rails itself will allow us all to focus our efforts and
follow a common goal.

This migration guide is rudimentary at best: it works for a `User`
object with one `avatar` attachment. It is provided as a framework to
start the documentation.

I have tried this in a dummy app.

[ActiveStorage]: http://edgeguides.rubyonrails.org/active_storage_overview.html