Media: Skip server-side scaling during client-side media processing

[itzmekhokan]

When an attachment is uploaded with generate_sub_sizes set to false, the client generates all derivatives itself — including the scaled full-size image it later supplies through the sideload endpoint. create_item() already suppressed thumbnail generation and EXIF rotation in this case, but not the "big image" downscaling gated by the big_image_size_threshold filter.

What the problem was:

• A large client-converted image (e.g. a HEIC converted to JPEG in the browser) was still scaled server-side to a -scaled file, which became the attached file while the untouched upload was recorded as original_image.
• The client's subsequent scaled sideload then collided with that -scaled file and was renamed -scaled-1; the thumbnails inherited the numbered name.
• The server-generated full-size file was left orphaned on disk and remained after the Media Library item was permanently deleted.

What the fix does:

• Disables big_image_size_threshold alongside the existing client-side processing filters in create_item(), and removes it again in remove_client_side_media_processing_filters().
• The uploaded full-size image is stored untouched, so the client's scaled sideload records it as original_image and keeps the plain -scaled name.

Approach and why:

• generate_sub_sizes = false is the contract that the client owns all derivative generation, which includes the scaled full-size image. Server-side scaling in that mode duplicates the client's work and causes the collision. Suppressing it is the minimal, consistent change — it sits directly beside the three filters already applied for the same purpose.

Trac ticket: https://core.trac.wordpress.org/ticket/65708

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[Copilot]

Pull request overview

This PR updates the REST attachments upload flow to better support client-side media processing when generate_sub_sizes=false, ensuring the server does not also perform “big image” downscaling (which can create -scaled filename collisions and leave stray files on disk).

Changes:

• Disable server-side big-image downscaling (big_image_size_threshold) during REST uploads when generate_sub_sizes is explicitly false.
• Ensure the new filter is removed alongside the existing client-side-processing-related filters.
• Add a PHPUnit REST controller test that reproduces the -scaled collision scenario and asserts the corrected behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/phpunit/tests/rest-api/rest-attachments-controller.php	Adds coverage verifying big-image scaling is skipped on upload when the client owns derivative generation, preventing -scaled collisions.
src/wp-includes/rest-api/endpoints/class-wp-rest-attachments-controller.php	Disables big_image_size_threshold during create_item() when generate_sub_sizes=false, and removes the filter during cleanup.

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