I'm using Apache POI to work with excel files. I'm trying to read an excel file (xlsx) and copy it to a new file (also xlsx) with a few rows and extra information added. This new file is then loaded into another web app in development by other people. The new file that is created is not being loaded by the web app, and it seems like the new file is missing something. For one thing, the file size is 8 kb, whereas the file size of the starting file is 53 kb, so I'm guessing when I'm writing the new file, I'm not adding some sort of information that is normally present in excel files. Any ideas?