Media: Re-introduce client-side media processing feature

[adamsilverstein]

Summary

• Reverts the removal in [62081] now that WordPress 7.1 has forked
• Restores all PHP functions, REST API endpoints, cross-origin isolation infrastructure, VIPS script module handling, build configuration, and associated tests
• 14 files restored, ~1,625 lines

Test plan

• CI passes (coding standards, PHPUnit, JS tests)
• Verify client-side media processing functions are restored in wp-includes/media.php
• Verify REST API endpoints for attachments are restored
• Verify cross-origin isolation test suite passes
• Verify VIPS script module registration is restored

Trac ticket: https://core.trac.wordpress.org/ticket/64919

Props adamsilverstein, jorbin, westonruter.
See #64906.

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[adamsilverstein]

This worked well in my testing. After this PR, the client side media worked just as it was working before the revert.

7.1 is now open for commits, see https://make.wordpress.org/core/2026/05/20/commence-operation-wp-7-1/

As soon as I commit this, I plan to work to backport code for a series of improvements that have already merged to Gutenberg. These backports were blocked because the code was missing from core.

Here is a complete list of PRs to backport:

1. Merged PRs — backports ready to merge (once restored)

These have landed on Gutenberg trunk and touch reverted Core code, so they are blocked only by
the restore. All merged after the 2026-03-20 removal.

PR	Merged	PHP touched	Core backport PR	Status / notes
#76731 — Add HEIC support using platform capabilities	2026-04-20	class-gutenberg-rest-attachments-controller.php, load.php (HEIC companion-file handling)	wordpress-develop#11323 — OPEN, drafted	Core PR also bundles #78128. Blocked by revert.
#78128 — Guard gutenberg_delete_heic_companion_file()	2026-05-12	load.php	Folded into wordpress-develop#11323	Hardening fix on top of #76731; rides the same Core PR.
#74903 — Add dimension validation to sideload endpoint	2026-05-15	class-gutenberg-rest-attachments-controller.php	wordpress-develop#11100 — OPEN, drafted	Validates dimensions on the sideload route. Blocked by revert.
#75888 — Enable concurrent sideload uploads	2026-04-20	class-gutenberg-rest-attachments-controller.php	adamsilverstein/wordpress-develop#48 — OPEN, drafted	Carries the Needs PHP backport label. No backport-changelog entry; open a Core PR.
#77036 — Deduplicate client-side image sizes with matching dimensions	2026-04-21	class-gutenberg-rest-attachments-controller.php	adamsilverstein/wordpress-develop#49 — OPEN, drafted (stacked on #48)	No backport-changelog entry and no backport label. Open a Core PR.
#77565 — Declare convert_format as boolean arg on sideload route	2026-04-23	class-gutenberg-rest-attachments-controller.php	adamsilverstein/wordpress-develop#50 — OPEN, drafted (stacked on #49; tests only, controller change already in core)	No backport-changelog entry and no backport label. Open a Core PR.

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2. Open PRs — draft the backport, keep it in draft until the GB PR merges

These touch reverted Core code but the Gutenberg PR is still open. Draft the Core PR if useful,
but keep it in draft until the Gutenberg PR lands (and the feature is restored).

PR	PHP touched	Core backport	Status / notes
#78410 — Add animated GIF to video conversion via mediabunny	animated-gif-to-video.php (new render-swap filter), class-gutenberg-rest-attachments-controller.php (animated-video / animated-video-poster sideload), client-assets.php, load.php	None yet	Carries Needs PHP backport. REST sideload roles depend on the CSM controller → blocked by revert. The editor also does GIF→video block-switching (core/video GIF variation); deletes attached files on media delete.
#77584 — Lazy-load JPEG XL (JXL) WASM on demand	class-gutenberg-rest-attachments-controller.php (finalize_item / sideload_item), load.php (adds JXL upload mimes; renames gutenberg_delete_heic_companion_file → gutenberg_delete_preserved_original_companion_file)	None yet	Reclassified: previously listed as JS-only, but it now touches the CSM controller and load.php → blocked by revert.
#78404 — Send Document-Isolation-Policy header on the site preview frame	load.php (cross-origin-isolation setup)	None yet	Depends on the COI scaffolding the revert removed → blocked. Related to #76662.
#76662 — Broaden DIP header scope to all admin pages	load.php (cross-origin-isolation setup)	None yet	Same COI scaffolding as #78404 → blocked. The two overlap; reconcile before backporting.
#76227 — WIP: Disable client-side media processing for non-Chromium browsers	lib/compat/wordpress-7.0/media.php (feature gate), load.php (Chromium version detection)	None yet	WIP. Reassess once finalized; the Chromium-detection logic may or may not be Core-bound.

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3. Watch list — related, but not blocked by the revert

PR	State	Why it's separate
#78420 — Return filtered wp_editor_set_quality in upload response	OPEN	Core backport drafted: wordpress-develop#11856 (OPEN). The image_quality field is added to the base WP_REST_Attachments_Controller schema / prepare_item_for_response, which exists in Core regardless of the revert — so not blocked by the restore. As an open PR, keep #11856 in draft until #78420 merges. Still waiting on a Trac ticket (core tests use placeholder @ticket 64906; replace once posted).
#75793 — Move image output format filtering to upload response	MERGED	Merged 2026-04-23 with the old No Core Sync Required bypass label. It does touch the CSM controller + load.php, so it likely owes a real Core backport once the feature is restored — reassess and open one.