Hi Stuff Worth Buying,
Yes, I suppose it is possible that you may have resaved the file in an incorrect format when you added more details through Excel. I also could not rule out the possibility that some of the data you added during that process may have made the file incompatible with our system.
We would need to take a look at the specific file you are having trouble with to definitively determine the source of the error and can do so if you file a support case from under the "Help" tab.
-Mike