Guys - how now brown cows!
This is so easy it hurts (lol) i.e. just move them to filesystem (so it generates the files accordingly) then copy the files so you have the "copy" of the attachments you desire. Of course the files will not have the proper names but will be viewable in an image editor. Once done downloading simply move them back into the database and done.
Super easy unless I missed something?
Edit: Ahh I see why you all thought... okay okay so basically yes it will be for example file names such as 34.attach and then the thumbnail would be 34.thumb which is clearly not 34.jpg or 34.png HOWEVER you can open these via photoshop OR a free program called Gimp 2 and they will open and display the file fine for viewing, you can then save to a proper windows style extension instead of vBulletins .thumb/.attach etc etc.