Media: Cover client-side big image scaling and orphaned files

[adamsilverstein]

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

The Gutenberg plugin already suppresses server-side "big image" downscaling while the client owns the derivatives (added in #75817), but nothing tested it. Every existing test in this area uses an image below the threshold, so a regression here would have gone unnoticed - which is roughly what happened in core, where the same filter never made it into WP_REST_Attachments_Controller. That gap is what @ianmjones ran into on the Trac ticket.

When the server scales the upload as well, three things go wrong at once:

• the client's own scaled file collides with the server's and is stored as -scaled-1
• the sub-sizes inherit the numbered name (-150x150-1 and so on)
• the server's full-size file is no longer referenced by the metadata, so it is orphaned on disk and survives "Delete Permanently"

This PR adds coverage for all three, and aligns the filter callback with the one core is adding in WordPress/wordpress-develop#12689 (__return_false, which is what the big_image_size_threshold docblock documents as the way to disable scaling). Same callback and priority means the plugin's registration and core's collapse into one instead of stacking.

I verified the tests catch the bug: with the plugin's filter removed, the e2e assertion fails on -scaled-1.jpeg and the PHP tests fail on big-photo-scaled.jpg / big-photo-150x150-1.jpg. With the filter in place they pass.

Note that the ticket describes this with a HEIC upload, but it is not HEIC specific - any image over the threshold hits it once the client owns the sub-sizes. The tests use a plain JPEG so they can run in Chromium, which has no native HEIC decode.

Why not a client-side change

The client is doing the right thing here. generate_sub_sizes: false is the contract that says the client owns every derivative including the scaled full-size copy, so the fix belongs on the server side of that contract. Core's PR does the same thing.

Testing instructions

Automated:

npm run test:unit:php -- --filter Gutenberg_REST_Attachments_Controller_Test
npm run test:e2e -- test/e2e/specs/editor/various/big-image-size-threshold.spec.js

To see them fail, comment out the big_image_size_threshold filter in Gutenberg_REST_Attachments_Controller::create_item() and re-run.

Manual:

1. In a browser with client-side media processing available (Chrome 137+), add an Image block.
2. Upload an image larger than 2560px on its longest side.
3. Check the attachment in the Media Library, or wp post meta get <id> _wp_attachment_metadata.
4. file should be <name>-scaled.jpg and original_image should be <name>.jpg, with sub-sizes named <name>-WxH.jpg. No -1 anywhere.
5. Delete the attachment permanently and confirm nothing is left in wp-content/uploads.

Types of changes

• Align the big_image_size_threshold filter callback with core's.
• Add PHP coverage for the upload not being scaled, and for the full upload/sideload/finalize flow leaving no orphaned files.
• Tighten the big image threshold e2e spec: assert exact file names instead of the conditional checks that were passing either way.

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Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>

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[github-actions]

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[adamsilverstein]

This is already fixed in core, no backport required.

[andrewserong]

Thanks for porting this to GB! Left a tiny nit-picky comment, feel free to ignore 😄

Tested in Playground and confirmed no unexpected -1 in the scaled filename in the media object returned by the REST API.

LGTM 👍

[andrewserong]

Totally optional: since we already use the exact same code in core, should we also re-use the same code comment?

Suggested change

	/*
	* Disable server-side "big image" downscaling; the client supplies
	* its own scaled version through the sideload endpoint. Scaling
	* here would create a conflicting `-scaled` file and orphan the
	* full-size upload. Uses the same callback and priority as
	* WP_REST_Attachments_Controller::create_item() so the two
	* registrations collapse into one on WordPress versions that
	* apply it themselves.
	*/
	add_filter( 'big_image_size_threshold', '__return_false', 100 );
	// Disable server-side "big image" downscaling; the client supplies its
	// own scaled version via the sideload endpoint. Scaling here would
	// create a conflicting "-scaled" file and orphan the full-size upload.
	add_filter( 'big_image_size_threshold', '__return_false', 100 );