i have not found a single example of using xbees for file transfers with checksum.
Widen your focus then. The issue is not HOW the data gets from point A to point B. It is how to be assured that A) the data got there, B) the data was what it was supposed to be, and C) the receiver is ready for more or needs the last packet resent.
All of those apply to any serial transmission. The XBees are just one way to accomplish serial transmission.
Look at Andrew Rapp's XBee library. It accomplishes all of those tasks.