[robelsust]

Description

In WP_REST_Attachments_Controller::prepare_item_for_response(), the image_output_format and image_quality fields directly called apply_filters( 'image_editor_output_format', array( $mime_type => $mime_type ), ... ).

Seeding the filter with array( $mime_type => $mime_type ) causes issues for plugins that add mappings using array union (+) rather than key assignment (e.g., add_filter( 'image_editor_output_format', fn( $formats ) => $formats + [ 'image/jpeg' => 'image/webp' ] )). Because $mime_type was already present as a key in the seeded array, the union operator would preserve the original MIME type instead of the plugin's mapped format.

This PR replaces those direct apply_filters() calls with wp_get_image_editor_output_format( $filename ? $filename : '', $mime_type ). This aligns the REST API behavior with WP_Image_Editor::get_output_format(), which also relies on wp_get_image_editor_output_format().

Changes

• Updated WP_REST_Attachments_Controller::prepare_item_for_response() in src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php to use wp_get_image_editor_output_format().
• Updated inline comments to correctly reference WP_Image_Editor::get_output_format().
• Added unit test test_image_output_format_with_array_union_filter() in tests/phpunit/tests/rest-api/rest-attachments-controller.php covering filters using array union (+).

Testing Instructions

1. Run PHPUnit tests:

   vendor/bin/phpunit --filter test_image_output_format

2. Verify all 5 tests pass without regressions:
   • test_image_output_format_and_progressive_defaults_in_create_response
   • test_image_output_format_with_custom_filter
   • test_image_output_format_with_array_union_filter
   • test_image_save_progressive_with_custom_filter
   • test_image_output_format_skipped_for_non_image
3. Run PHPCS to verify coding standards:

   vendor/bin/phpcs src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php tests/phpunit/tests/rest-api/rest-attachments-controller.php

Trac ticket: https://core.trac.wordpress.org/ticket/64677

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