Scheduled backup should only overwrite previous backup when the process completes

Scenario:

1. Backup file name is customized so as not to contain current date in the file name, for example bamboo_backup.zip instead of bamboo_backup_20150624.zip
2. Using scheduled backups, not manual backups (manual backups don't allow overwriting, a message "File already exists" shows up below the file name text field).

Problem:

1. When the scheduled backup process starts, if there is already a file named bamboo_backup.zip in BAMBOO_HOME/exports, this file will be deleted by Bamboo, and then a new file will be created and Bamboo will start writing data into it.
2. If something bad happens (e.g., power outage, server reboot, JVM crash) while the backup process is running, the backup file will be corrupt, and there will be no healthy backups.

Suggestion:
Bamboo should only overwrite the existing file when Bamboo finishes writing into the new backup file.