wp_ajax_media_create_image_subsizes

function wp_ajax_media_create_image_subsizes() {
	check_ajax_referer( 'media-form' );

	if ( ! current_user_can( 'upload_files' ) ) {
		wp_send_json_error( array( 'message' => __( 'Sorry, you are not allowed to upload files.' ) ) );
	}

	if ( empty( $_POST['attachment_id'] ) ) {
		wp_send_json_error( array( 'message' => __( 'Upload failed. Please reload and try again.' ) ) );
	}

	$attachment_id = (int) $_POST['attachment_id'];

	if ( ! empty( $_POST['_wp_upload_failed_cleanup'] ) ) {
		// Upload failed. Cleanup.
		if ( wp_attachment_is_image( $attachment_id ) && current_user_can( 'delete_post', $attachment_id ) ) {
			$attachment = get_post( $attachment_id );

			// Created at most 10 min ago.
			if ( $attachment && ( time() - strtotime( $attachment->post_date_gmt ) < 600 ) ) {
				wp_delete_attachment( $attachment_id, true );
				wp_send_json_success();
			}
		}
	}

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

	// This can still be pretty slow and cause timeout or out of memory errors.
	// The js that handles the response would need to also handle HTTP 500 errors.
	wp_update_image_subsizes( $attachment_id );

	if ( ! empty( $_POST['_legacy_support'] ) ) {
		// The old (inline) uploader. Only needs the attachment_id.
		$response = array( 'id' => $attachment_id );
	} else {
		// Media modal and Media Library grid view.
		$response = wp_prepare_attachment_for_js( $attachment_id );

		if ( ! $response ) {
			wp_send_json_error( array( 'message' => __( 'Upload failed.' ) ) );
		}
	}

	// At this point the image has been uploaded successfully.
	wp_send_json_success( $response );
}