The existing filter `image_editor_output_format` receives an additional parameter `$size_name` which is populated whenever it controls the output format for a specific registered image size to create. Otherwise, it remains empty. In order to achieve this, a low level change has been added in bringing a new `$size_name` class property to the `WP_Image_Editor` base class, which is introduced in a backward compatible way that will not cause conflicts with custom implementations.
This parameter is then used in new logic inside the `wp_default_image_output_mapping()` callback function for the filter, controlling whether `image/jpeg` should map to `image/webp` output or not. By default, this is enabled for all WordPress core image sizes by default, and this list can be modified using a new `wp_image_sizes_with_additional_mime_type_support` filter, e.g. to remove core sizes or add custom sizes.
The customization per image size may be further enhanced by providing a more declarative API via a new parameter on the `add_image_size()` function.
Props eugenemanuilov, flixos90, adamsilverstein, joegrainger.
Fixes#56526.
See #55443, #56288.
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