Ajoutez ce code dans le fichier functions.php :

function seomix_adm_media_attachment_alt($cols) {
	$cols["alt"] = 'ALT Text';
	return $cols;
}
function seomix_adm_media_attachment_alt_content($column_name, $id) {
	if ( $column_name === 'alt' ) {
		echo get_post_meta( $id, '_wp_attachment_image_alt', true);
	}
}
add_filter('manage_media_columns', 'seomix_adm_media_attachment_alt', 1);
add_action('manage_media_custom_column', 'seomix_adm_media_attachment_alt_content', 1, 2);