Modify the MIME types of the Starter Templates

mime.php

<?php
if ( ! function_exists( 'prefix_modify_mime_type' ) ) :

	/**
	 * Modify the MIME types.
	 *
	 * @since x.x.x
	 *
	 * @param  array  $args 	File upload arguments.
	 * @return array.
	 */
	function prefix_modify_mime_type( $args ) {

		$args['mimes']['json'] = 'text/plain';

		return $args;
	}
	add_filter( 'ast_block_templates_wp_handle_sideload', 'prefix_modify_mime_type' );

endif;