#!/bin/bash
s='[2021-05-14 11:15:18 CEST]     0B files1/
[2021-05-14 11:15:18 CEST]     0B files2/
[2021-05-14 11:15:19 CEST]     0B file1.ext
[2021-05-14 11:15:18 CEST]     0B file2.ext'

sed -n 's/^\[[^][]*][[:blank:]]*[^[:blank:]]*[[:blank:]]\(.*\.ext\)$/\1/p' <<< "$s"