Media: Enable HEIC/HEIF uploads when server lacks image editor support

[adamsilverstein]

Trac ticket: https://core.trac.wordpress.org/ticket/64915

What?

Allow HEIC/HEIF image uploads to succeed even when the server's image editor (ImageMagick/GD) doesn't support HEIC. Currently, wp_prevent_unsupported_mime_type_uploads blocks these uploads entirely.

Gutenberg PR: WordPress/gutenberg#76731

Why?

HEIC is the default photo format on iPhones. When users upload HEIC images and the server can't process them, the upload fails with an unhelpful error. With the client-side media processing feature, the browser can decode HEIC using its native createImageBitmap() API (leveraging OS-licensed HEVC codecs) and convert to JPEG for sub-size generation. But first, the server needs to accept the upload.

How?

In WP_REST_Attachments_Controller::create_item_permissions_check(), bypass the $prevent_unsupported_uploads check when the uploaded file is HEIC/HEIF (detected via the existing wp_is_heic_image_mime_type() helper). This allows the file to be stored on the server, after which the client-side canvas fallback generates a JPEG version and all required sub-sizes.

Testing Instructions

1. Use a WordPress environment where ImageMagick does NOT have HEIC support
2. Upload a HEIC image via the REST API media endpoint
3. Verify the upload succeeds (previously would return rest_upload_image_type_not_supported error)
4. Verify missing_image_sizes is populated in the response (server couldn't generate sub-sizes)

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[github-actions]

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