Media: skip server image support check when not generating sub-sizes

[adamsilverstein]

Summary

REST API: Skip server-side image editor support check when generate_sub_sizes is false

Description

When uploading images via the REST API with generate_sub_sizes set to false, the server still checks whether the image editor supports the uploaded image format. This causes uploads of formats like AVIF to fail with rest_upload_image_type_not_supported, even though the client is handling all image processing and the server doesn't need to generate any sub-sizes.

The generate_sub_sizes parameter was introduced to allow clients (such as the block editor with client-side media processing) to handle image resizing themselves. When this parameter is false, the server-side image editor support check is unnecessary and should be skipped.

Steps to Reproduce

1. Enable client-side media processing in the block editor (or use the REST API directly with generate_sub_sizes=false)
2. Attempt to upload an AVIF image on a server where the image editor does not support AVIF
3. The upload fails with rest_upload_image_type_not_supported

Expected Result

The upload should succeed because the client is handling all image processing.

Actual Result

The upload fails with a 400 error: "The web server cannot generate responsive image sizes for this image."

Proposed Fix

In WP_REST_Attachments_Controller::create_item_permissions_check(), after the wp_prevent_unsupported_mime_type_uploads filter is applied, check if generate_sub_sizes is explicitly set to false. If so, set $prevent_unsupported_uploads to false to bypass the server-side image editor support check.

## Summary

When uploading images via the REST API with generate_sub_sizes set to false, the server still checks whether the image editor supports the uploaded image format. This causes uploads of formats like AVIF to fail with rest_upload_image_type_not_supported, even though the client is handling all image processing and the server doesn't need to generate any sub-sizes.

This PR adds a check in WP_REST_Attachments_Controller::create_item_permissions_check() to skip the server-side image editor support check when generate_sub_sizes is explicitly false.

Trac ticket: https://core.trac.wordpress.org/ticket/64836

## Changes

• class-wp-rest-attachments-controller.php: After the wp_prevent_unsupported_mime_type_uploads filter is applied, check if generate_sub_sizes is false and bypass the image editor support check if so.
• rest-attachments-controller.php (tests): Two new tests:
  • test_upload_unsupported_image_type_skipped_when_not_generating_sub_sizes — verifies upload succeeds with generate_sub_sizes=false
  • test_upload_unsupported_image_type_enforced_when_generating_sub_sizes — verifies upload still fails with generate_sub_sizes=true

## Related

• Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/76371
• Gutenberg Issue: ​https://github.com/WordPress/gutenberg/issues/76369
• Trac: https://core.trac.wordpress.org/ticket/62717

## Test Plan

• [ ] Run PHP tests: vendor/bin/phpunit tests/phpunit/tests/rest-api/rest-attachments-controller.php --filter="test_upload_unsupported_image_type"
• [ ] Verify the two new tests pass and existing tests still pass

## Proposed SVN Commit Message

Media: Skip server image support check when not generating sub-sizes.

When uploading images via the REST API with `generate_sub_sizes` set to `false`, the server still checks whether the image editor supports the uploaded image format. This causes uploads of formats like AVIF to fail with `rest_upload_image_type_not_supported`, even though the client is handling all image processing and the server doesn't need to generate any sub-sizes.

Skip the server-side image editor support check in `WP_REST_Attachments_Controller::create_item_permissions_check()` when `generate_sub_sizes` is explicitly `false`.

See also https://github.com/WordPress/gutenberg/issues/76369 and https://github.com/WordPress/gutenberg/pull/76371.

Props adamsilverstein, westonruter, andrewserong.
Fixes #64836. See #62717.

## Proposed SVN Commit Message

Media: Skip server image support check when not generating sub-sizes.

When uploading images via the REST API with `generate_sub_sizes` set to `false`, the server still checks whether the image editor supports the uploaded image format. This causes uploads of formats like AVIF to fail with `rest_upload_image_type_not_supported`, even though the client is handling all image processing and the server doesn't need to generate any sub-sizes.

Skip the server-side image editor support check in `WP_REST_Attachments_Controller::create_item_permissions_check()` when `generate_sub_sizes` is explicitly `false`.

See also https://github.com/WordPress/gutenberg/issues/76369 and https://github.com/WordPress/gutenberg/pull/76371.

Props adamsilverstein, westonruter, andrewserong.
Fixes #64836. See #62717.

Mark ready for commit

[adamsilverstein]

In 61980:

Media: skip server image support check when using client-side media.

When uploading images via the REST API with generate_sub_sizes set to false, skip server support checks since processing is handled on the client side. Fix an issue where uploads of formats like AVIF fail with rest_upload_image_type_not_supported, even though the client will handle all image processing and the server doesn't need to support the format.

See also ​https://github.com/WordPress/gutenberg/issues/76369 and ​https://github.com/WordPress/gutenberg/pull/76371.

Props adamsilverstein, westonruter, andrewserong, mukesh27.
Fixes #64836. See #62717.