Is it possible to update filefield link(remapping) instead of uploading the file.
If existing file is say screenshot-1.jpg then ,
token can be set in place of replacement link field say [node:field-image:url]-[site-name]
would change the filefield value so that it will remapped to screenshot-1-drupal.org.jpg
and yes special care should be taken for retaining file extension ie replacement should be done to filename without extension.
In actual filesystem bulk rename of files will be done thorough terminal command as it is faster.
This feature can extentedn to alttext too e.x.
original alt text - This is a private image
token [node:field-image:url]-(approved by - [site:current-user])
remapped into file This is a private image(approved by-Jonny).
This feature will also be useful when migrating from wamp to lamp where system is case sensitive.