[Mailer] Remove line breaks in email attachment content with Sendgrid API

[fyfey]

Line breaks are not allowed in attachment content when sending over the
API.

Q	A
Branch?	4.3
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	Fix #33671, Closes #32645
License	MIT
Doc PR

This is a fix for #33671. Send grid's API throws a 400 error when sending email attachments with default base64 encoding.
Removing the line breaks resolves this issue.

[stof]

There is no line-break in this content, so this is not testing your patch.

[chalasr]

@stof it does, line-breaks are added by the MIME-compliant base64 encoder:

symfony/src/Symfony/Component/Mime/Encoder/Base64Encoder.php

Line 39 in 7788612

return $firstLine.trim(chunk_split($encodedString, $maxLineLength, "\r\n"));

[javiereguiluz]

This is so confusing that we should add a comment before the ->attach() method. Something like:

// even it content doesn't include new lines, the Base64 encoding performed later may add them
->attach('...)

[javiereguiluz]

This is so confusing that we should add a comment before the ->attach() method. Something like:

// even it content doesn't include new lines, the Base64 encoding performed later may add them
->attach('...)

[stof]

Maybe the actual issue is the usage of bodyToString on the DataPart, relying on the base64 encoding built for SMTP while this is sent to an HTTP API.

Btw, this may also impact other API providers.

[fabpot]

@stof is right. We had the same kind of issue with SmtpEnvelope which was optimized for SMTP. We fixed that in 4.4 by renaming it to Envelope but more importantly moved the SMTP logic to the SMTP classes. Maybe something we need to do here as well. I haven't looked at the code to have an idea on how to do it properly though.

[chalasr]

The SendGrid HTTP API requires attachments to be base64 encoded though (but with "strict" base64 i.e. no line-break). But API transports could indeed bypass the encoding system and use base64_encode() directly if needed, that's what I propose in #32645 for the very same bug.

[nicolas-grekas]

Thank you @fyfey.