Folks, there are many threads on this board tracking the same issue with the corrupted file error received upon ios 6.1 upgrade for iPad 2 users. I was able to fix the problem based on a solution posted in this thread > https://discussions.apple.com/message/21074848#21074848 I hope you find this helpful and good luck!!
On Windows XP :
Documents and Settings\<User>\Application Data\Apple Computer\iTunes\iPhone Software Updates
On Windows Vista/Windows 7:
Users\<User>\AppData\Roaming\Apple Computer\iTunes\iPhone Software Updates
~/Library/iTunes/iPad Software Updates
Delete the software update for the iPad and then try restoring.
I am using a G5 PowerMac, OS 10.5.8. A co-worker experienced this same problem. My solution was:
1) at Redmond Pie http://www.redmondpie.com/download-ios-6.1-final-for-iphone-ipad-and-ipod-touch- direct-links/ I downloaded the iPad2,1_6.1_10B141_Restore.ipsw
2) moved that file to Users/(Me)/Library/iTunes/iPad Software Updates
3) launched iTunes and connected the iPad
4) at the prompt (iTunes has detected an iPad in Recovery mode...) click "OK"
5) click "Restore"
This circumvents the downloading portion of the update process, which is apparently where the corruption occurs.
I'll hand the iPad back to my co-worker tomorrow and she can sync with her latest back-up (let this be a lesson to those who don't get at least one fairly recent and accessable back-up- the Cloud isn't always available without first re-entering a lot of info)
Hope this helps- my research indicates a widespread and frustrating problem here and the Apple Geniuses are a little behind on the solution