REST API: Extend orderby parameter to support mime_type for attachments.

[ramonjd]

This update allows the orderby request argument in the attachments controller to include mime_type, enhancing the sorting capabilities of media items.

Trac ticket: https://core.trac.wordpress.org/ticket/64073

Testing

npm run test:php -- --filter WP_Test_REST_Attachments_Controller

In the block editor:

// desc by default, e.g., video, audio
await wp.data.resolveSelect( 'core' ).getEntityRecords( 'postType', 'attachment', { orderby: 'mime_type'  } );

// asc, e.g., audio, video
await wp.data.resolveSelect( 'core' ).getEntityRecords( 'postType', 'attachment', { orderby: 'mime_type', order: 'asc'  } );

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[ramonjd]

Before I add tests (in Tests_Query_Results I think(, I'd like to check if adding post_mime_type to this whitelist is okay?

The alternative is to add a filter in the attachments controller I guess, e.g.,

// In prepare_items_query method, after the orderby mapping:
if ( isset( $query_args['orderby'] ) && isset( $request['orderby'] ) && 'mime_type' === $request['orderby'] ) {
    // Use posts_orderby filter to add custom ordering
    add_filter( 'posts_orderby', array( $this, 'orderby_mime_type' ), 10, 2 );
    // Set orderby to something that won't conflict
    $query_args['orderby'] = 'date';
}

public function orderby_mime_type( $orderby, $query ) {
    global $wpdb;
    $order = $query->get( 'order' );
    return "{$wpdb->posts}.post_mime_type {$order}";
}

[ramonjd]

@TimothyBJacobs made a good point about a long-standing pagination bug

This change might make that bug more obvious, not convinced it's a blocker thought.

It does make me lean into using a filter in the controller thought as we can set a deterministic field like id to get around the pagination bug.

[andrewserong]

Before I add tests (in Tests_Query_Results I think(, I'd like to check if adding post_mime_type to this whitelist is okay?

IMO because post_mime_type is a field on the wp_posts table, I think including it in this allow list makes sense, rather than adding a filter to the attachments controller. But I'm not very experienced with the WP_Query class, so feel free to get a second opinion!

made a good point about a long-standing pagination bug

That is a good point. I think it's a really important bug to fix as there's going to be lots of situations where it'll crop up (i.e. order by post_author). But as you mention, this issue exists already, and there are lots of other fields where this situation can come up, so I wouldn't think of it as a blocker to this particular PR, either.