https://buildkite.com/rails/rails/builds/71675#2b9c36e5-5504-4a8e-8c4d-af774e96ea16/954-982
https://buildkite.com/rails/rails/builds/71675#2b9c36e5-5504-4a8e-8c4d-af774e96ea16/954-988

```bash
.................DEPRECATION WARNING: Passing a path to `fixture_file_upload`
relative to `fixture_path` is deprecated.
In Rails 6.2, the path needs to be relative to `file_fixture_path`.

Please modify the call from
`fixture_file_upload("files/avatar1.jpeg")` to `fixture_file_upload("avatar1.jpeg")`.
DEPRECATION WARNING: Passing a path to `fixture_file_upload` relative to `fixture_path` is deprecated.
In Rails 6.2, the path needs to be relative to `file_fixture_path`.

Please modify the call from
`fixture_file_upload("files/avatar2.jpeg")` to `fixture_file_upload("avatar2.jpeg")`.
........................................................................
```

`fixture_file_upload` now uses path relative to `file_fixture_path`, see
https://github.com/rails/rails/pull/39086,
https://github.com/rails/rails/blob/61c4be477706b721688e36a3168f86aabc625658/actionmailbox/test/test_helper.rb#L21

Sendgrid, like Mailgun, only passes BCC recipients as a parameter in the original JSON payload.

This PR adds code to prepend the recipients from the Sendgrid payload to the raw_email under the X-Original-To header.

References #38738.

Generators generate things, but what is meant by 'Stubbing out' might
confuse beginners and non-native English speakers.
While generated tests are stubs that should have an implementation, a
generated model is a valid model that doesn't require any changes.

- Add the configuration option for annotating templates with file names to the generated app.
- Add `annotate_rendered_view_with_filenames` option to configuring guide.

Mandrill's Inbound API checks to see if a URL exists before it creates
the webhook. It sends a HEAD request, to which we now return a 200 OK
response to indicate that the route exists.

Now we can generate inbound API calls with ease on Mandrill, without
having to shuffle around tokens in production.

Fixes #37609.

* Add a way to deliver inbound emails by source

Great for testing when you get an eml file and can just paste it in.

* Test updates

* Fix tests

* Fix spacing

As a developer, when looking at a page in my web browser, it's sometimes
difficult to figure out which template(s) are being used to render the page.

config.action_view.annotate_template_file_names adds HTML comments to the
rendered output indicating where each template begins and ends.
Co-authored-by: NAaron Patterson <tenderlove@github.com>

* require, require_relative, load by double quotes

We're getting rid of all single quote usage, unless it serves a specific purpose, as per the general style guide.

Expose the Mailgun SMTP recipient to Action Mailbox as X-Original-To, like Postfix.

Allows handling BCCed recipients who aren't listed in To/CC fields.

Mailgun now needs the signing key instead of the API key for validating incoming mails.

Follow-up to #37053.  The `*.sqlite3-*` .gitignore entries added in that
commit subsume previous `*.sqlite3-journal` entries.

Forward ActionMailbox install to install generator and hide the ActionMailbox install generator from generator list

Updated the Action Mailbox install task description

The from_address method is added to Mail::Message, but is not used
internally to ActionMailbox, nor tested (even implicitly). As it is part
of the public API, it feels important to at least have a basic test of
it.

Similarly, the x_original_to_addresses was only implicitly tested via
the recipients_addresses method. Given other similar methods are
explicitly tested, it feels like an oversight not to test it as well.

As the test introduced from_address didn't align with the naming of
RecipientsTest, I renamed it to the more generic AddressesTest.

- `a Active Storage` -> `an Active Storage`
- `which be uploaded` -> `which is uploaded`

Rails 6.0 introduces parallel testing and the default degree of parallelism is configured based on the number of CPU.
refer https://github.com/rails/rails/pull/34735 https://github.com/rails/rails/pull/31900

When any minitest executed under the OS where 2 or more CPU available,
SQLite 3 database files are not git-ignored.

Also updated other files which ignores SQLite database files.

* Steps to reproduce

```
$ git clone https://github.com/yahonda/rep_ignore_sqlite3_databases.git
$ cd rep_ignore_sqlite3_databases/
$ bin/rails test
$ git status
```

* Expected behavior:

- No `Untracked files:`

* Actual behavior:

- SQLite 3 database files appeared

```
$ git status
On branch master
Untracked files:
  (use "git add <file>..." to include in what will be committed)

	db/test.sqlite3-0
	db/test.sqlite3-1
	db/test.sqlite3-2
	db/test.sqlite3-3
	db/test.sqlite3-4
	db/test.sqlite3-5

nothing added to commit but untracked files present (use "git add" to track)
$
```

Allow passing a block to the create_inbound_email_from_mail and
receive_inbound_email_from_mail test helper methods.

When you wanted to test a multipart email -- for example, an email that
has both a plaintext part and an HTML part -- there wasn't a way to
easily build one without a pre-made fixture. By allowing you to pass a
block to these methods, we unlock the power of Mail.new to easily
create a variety of emails, from multi-part emails, to emails with
attachments, and beyond.

controller passed along attachment file paths instead of their filenames