WP_REST_Attachments_Controller::create_item

	 * @since 4.7.0
	 *
	 * @param WP_REST_Request $request Full details about the request.
	 * @return WP_REST_Response|WP_Error Response object on success, WP_Error object on failure.
	 */
	public function create_item( $request ) {
		if ( ! empty( $request['post'] ) && in_array( get_post_type( $request['post'] ), array( 'revision', 'attachment' ), true ) ) {
			return new WP_Error(
				'rest_invalid_param',
				__( 'Invalid parent type.' ),
				array( 'status' => 400 )
			);
		}

		$insert = $this->insert_attachment( $request );

		if ( is_wp_error( $insert ) ) {
			return $insert;
		}

		$schema = $this->get_item_schema();

		// Extract by name.
		$attachment_id = $insert['attachment_id'];
		$file          = $insert['file'];

		if ( isset( $request['alt_text'] ) ) {
			update_post_meta( $attachment_id, '_wp_attachment_image_alt', sanitize_text_field( $request['alt_text'] ) );
		}

		if ( ! empty( $schema['properties']['meta'] ) && isset( $request['meta'] ) ) {
			$meta_update = $this->meta->update_value( $request['meta'], $attachment_id );

			if ( is_wp_error( $meta_update ) ) {
				return $meta_update;
			}
		}

		$attachment    = get_post( $attachment_id );
		$fields_update = $this->update_additional_fields_for_object( $attachment, $request );

		if ( is_wp_error( $fields_update ) ) {
			return $fields_update;
		}

		$request->set_param( 'context', 'edit' );

		/**
		 * Fires after a single attachment is completely created or updated via the REST API.
		 *
		 * @since 5.0.0
		 *
		 * @param WP_Post         $attachment Inserted or updated attachment object.
		 * @param WP_REST_Request $request    Request object.
		 * @param bool            $creating   True when creating an attachment, false when updating.
		 */
		do_action( 'rest_after_insert_attachment', $attachment, $request, true );

		if ( defined( 'REST_REQUEST' ) && REST_REQUEST ) {
			// Set a custom header with the attachment_id.
			// Used by the browser/client to resume creating image sub-sizes after a PHP fatal error.
			header( 'X-WP-Upload-Attachment-ID: ' . $attachment_id );
		}

		// Include media and image functions to get access to wp_generate_attachment_metadata().
		require_once ABSPATH . 'wp-admin/includes/media.php';
		require_once ABSPATH . 'wp-admin/includes/image.php';

		// Post-process the upload (create image sub-sizes, make PDF thumbnails, etc.) and insert attachment meta.
		// At this point the server may run out of resources and post-processing of uploaded images may fail.
		wp_update_attachment_metadata( $attachment_id, wp_generate_attachment_metadata( $attachment_id, $file ) );

		$response = $this->prepare_item_for_response( $attachment, $request );
		$response = rest_ensure_response( $response );