[wp-trac] [WordPress Trac] #15448: wp_mail() sets Content-Type header twice for multipart emails

[WordPress Trac]

#15448: wp_mail() sets Content-Type header twice for multipart emails
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 Reporter:  rmccue                               |       Owner:  SirLouen
     Type:  defect (bug)                         |      Status:  closed
 Priority:  high                                 |   Milestone:  6.9
Component:  Mail                                 |     Version:  2.8
 Severity:  normal                               |  Resolution:  fixed
 Keywords:  has-unit-tests has-test-info commit  |     Focuses:
  needs-patch                                    |
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Changes (by rezwanshiblu1952):

 * keywords:  has-patch has-unit-tests has-test-info commit => has-unit-
     tests has-test-info commit needs-patch


Comment:

 Test Report:
 This indicated the patch works as expected.
 ----

 Patch tested:
 https://github.com/WordPress/wordpress-develop/pull/9500.patch
 ----

 Environment:
 WordPress: 6.9-alpha-60093-src
 PHP: 8.3
 Server: apache
 Database: mysqli
 OS: MacOS
 Theme: Twenty Twenty-Five
 MU Plugins: None activated
 Plugins:
 Amazon S3 Email Testing 1.0.0
 Manual Multipart/Alternative/Mixed Test 1.0.0
 Test Reports 1.2.0
 WP Mail SMTP 4.6.0
 ----

 Actual Results:
 1. Emails sent through wp_mail() with multipart formatting are now
 properly interpreted.
 2. Mailpit correctly separates and displays the plain text and HTML
 sections in their respective tabs.
 3. File attachments (e.g., attachment.pdf) are now detected and can be
 accessed as expected.
 4. MIME headers and boundaries are no longer exposed within the message
 body.
 ----

 Prior to applying the patch, Mailpit rendered the raw multipart message
 structure, including inline MIME headers, and failed to recognize
 attachments. After the patch, Mailpit correctly processes multipart
 messages in line with PHPMailer’s expected behavior. No issues or
 regressions were detected in the core wp_mail() functionality.

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Ticket URL: <https://core.trac.wordpress.org/ticket/15448#comment:101>
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