[Mailer] [Scaleway] Fix attachment handling

[madbob]

Q	A
Branch?	6.4
Bug fix?	yes
New feature?	no
Deprecations?	no
License	MIT

Here a pair of fixes for attachments' handling.

1. the attribute in the JSON payload is "attachments", not "attachment". Here you find the reference documentation

2. as bodyToString() already returns a base64-encoded string, there is no need to re-encode it. I've replicated the behaviour adopted by BrevoMailer, that I also use with profit

[carsonbot]

Hey!

I see that this is your first PR. That is great! Welcome!

Symfony has a contribution guide which I suggest you to read.

In short:

• Always add tests
• Keep backward compatibility (see https://symfony.com/bc).
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I am going to sit back now and wait for the reviews.

Cheers!

Carsonbot

[OskarStark]

Thank you. Can you please add or extend an existing testcase to avoid further regressions? Thank you

[madbob]

Done. I've extended the existing test to check the output of attachment's serialization.

[nicolas-grekas]

Thank you @madbob.