Source

File: wp-admin/includes/file.php

function wp_get_theme_file_editable_extensions( $theme ) {

	$default_types = array(
		'bash',
		'conf',
		'css',
		'diff',
		'htm',
		'html',
		'http',
		'inc',
		'include',
		'js',
		'json',
		'jsx',
		'less',
		'md',
		'patch',
		'php',
		'php3',
		'php4',
		'php5',
		'php7',
		'phps',
		'phtml',
		'sass',
		'scss',
		'sh',
		'sql',
		'svg',
		'text',
		'txt',
		'xml',
		'yaml',
		'yml',
	);

	/**
	 * Filters the list of file types allowed for editing in the theme editor.
	 *
	 * @since 4.4.0
	 *
	 * @param string[] $default_types An array of editable theme file extensions.
	 * @param WP_Theme $theme         The current theme object.
	 */
	$file_types = apply_filters( 'wp_theme_editor_filetypes', $default_types, $theme );

	// Ensure that default types are still there.
	return array_unique( array_merge( $file_types, $default_types ) );
}