#!/bin/bash
s='<link rel="colorSchemeMapping
"
href="marinedrugs-790193-original.fld/colorschememapping.xml">'
sed -E ':a;N;$!ba;s/\n(")\n|\n/\1 /g' <<< "$s"
IyEvYmluL2Jhc2gKcz0nPGxpbmsgcmVsPSJjb2xvclNjaGVtZU1hcHBpbmcKIgpocmVmPSJtYXJpbmVkcnVncy03OTAxOTMtb3JpZ2luYWwuZmxkL2NvbG9yc2NoZW1lbWFwcGluZy54bWwiPicKc2VkIC1FICc6YTtOOyQhYmE7cy9cbigiKVxufFxuL1wxIC9nJyA8PDwgIiRzIgo=