Upload media (image) and set the category

Question

I want to upload an image and attach to it a category. This is my code:

function upload_cover(WP_REST_Request $request) {

    require_once( ABSPATH . 'wp-admin/includes/image.php' );
    require_once( ABSPATH . 'wp-admin/includes/file.php' );
    require_once( ABSPATH . 'wp-admin/includes/media.php' );

    $attachment_id = media_handle_upload('poster', 0);
    $event = array(
        'post_status' => 'publish',
        'post_type'   => 'poster',
        'meta_input'  => array(),
        'post_category' => array('poster')
    );
    $post_id = wp_insert_post( $event );
    wp_set_post_terms($post_id, 'poster', 'category');
}

The uploader works fine, but no category is attached to the image. I have tried and with these:

$cat_id = get_cat_ID('cover');

add_term_meta( $cat_id, 'poster', $post_id, true );
wp_set_post_categories($post_id, array('poster'), true);
wp_set_post_terms($post_id, array('poster'), 'category');

For the categories for images, I am using this plugin Media Library Categories.

Answer 1

First of all, if you want to apply categories to Attachments, you have to enable categories for the attachment.

You can do this by using the register_taxonomy_for_object_type() function. In your plugin file or theme functions file, add the following:

function wp_add_categories_to_attachments() {
    register_taxonomy_for_object_type( 'category', 'attachment' );
}
add_action( 'init' , 'wp_add_categories_to_attachments' );

Answer 2

This fix the problem: wp_set_object_terms($attachment_id, 'poster', 'category', true);

https://developer.wordpress.org/reference/functions/wp_set_object_terms/