System.IO.Packaging is easy to work with because the resulting files are ZIPped folders, so you can simply rename the file to .zip and then open it. I do this with XLSX files to see how they are built.
Unfortunately, the reverse is not true; if you re-compress the folder and rename it .XLSX, Excel with say it has unreadable content. This is true even if the file is unchanged. If you simply rename, extract all, recompress, rename, the resulting file does not work.
Poking about inside, it seems the format of the file is not a canonical zip, header information is missing?
Anyone know an easy trick to recompress these files in Explorer? It would be nice to be able to test changes.
