</Solidjeuh> identified this in https://www.phpbb.com/community/viewtopic.php?f=556&t=2510936
Selecting an otherwise successful database backup for restore always results in "The selected file to backup is invalid" when attempting to confirm and proceed with the actual restore.
This appears to have been introduced during the https://tracker.phpbb.com/browse/PHPBB3-15928 changes in phpBB 3.2.6 and later. User KillBill. had debug enabled, which catches that /includes/acp/acp_database.php line 219 attempts to reference array index "extensions", instead of the "extension" index that exists in backup_info.
Correcting line 219 to reference "extension" has allowed the restore to successfully proceed.