Messages with attachments are rejected by the SendGrid API

Problem/Motivation

Sending a message that has a file attachment fails. The SendGrid API rejects the request with a 400 and this response:

{ "errors": [ { "message": "If present, text/plain and text/html may only be provided once.", "field": "content" } ] }

The message is never queued, so nothing is delivered.

SendGridMail::mail() builds the request by reading the message's Content-Type header. When the type is multipart/mixed, which is what Mime Mail produces whenever a message carries an attachment, it splits the body on the boundary and loops over every part. For each part it tests whether the text "text/plain" or "text/html" appears anywhere in that part, and if it does, adds the part to the request as content:

  foreach ($body_parts as $body_part) {
    if (strpos($body_part, 'multipart/alternative')) {
      // Splits the nested part and adds text/plain and text/html from it.
    }
    else {
      if (strpos($body_part, 'text/plain')) {
        $sendgrid_message->addContent('text/plain', ...);
      }
      elseif (strpos($body_part, 'text/html')) {
        $sendgrid_message->addContent('text/html', ...);
      }
    }
  }

The test is a plain substring search across the whole part, including its headers and its encoded body, and the loop visits attachment parts as well as message parts. Nothing prevents the same content type being added twice, and the SendGrid API accepts at most one part of each type.

Attachments are added separately, from $message['params']['attachments'], so any content the body parser matches after the first of a given type is surplus regardless of where it came from.

This is present on 8.x-2.3 and on the current 8.x-2.x branch.

Steps to reproduce

1. Install and configure Mail System with Mime Mail as the formatter and SendGrid Integration as the sender.
2. Send any message that includes a file attachment, passed in $message['params']['attachments'].
3. The send fails. /admin/reports/dblog records a sendgrid_integration error containing "Sending emails to Sendgrid service failed with error code 400" and a second entry with the API response quoted above.

Sending the same message without an attachment succeeds, because Mime Mail then produces multipart/alternative or text/html rather than multipart/mixed, and those take different branches that cannot match more than once.

Proposed resolution

Add each content type at most once, keeping the first match.

Introduce a small helper, addContentOnce(), and route all eleven text/plain and text/html additions in SendGridMail::mail() through it, with a local array tracking which types have already been added.

First match wins is the correct precedence: Mime Mail places the multipart/alternative block holding the real message body ahead of the attachment parts, so the genuine content is the one used.

This deliberately does not change how the parser decides which parts are message content and which are attachments. That would be a larger change with more scope to alter behaviour for other senders, and it is not necessary to fix this: because attachments are added from $message['params']['attachments'], anything the body parser matches beyond the first of a type is surplus by definition.

A merge request against 8.x-2.x will follow.

Remaining tasks

- Open a merge request against 8.x-2.x
- Review
- Confirm on a live send with an attachment
- Consider whether a test covering a multipart/mixed message with an attachment is wanted

User interface changes

None.

API changes

Adds one protected method, SendGridMail::addContentOnce(). No existing method signature or behavior changes for callers. Subclasses overriding mail() are unaffected.

Data model changes

None.