Turns out we’ve got an excel spread sheet (.xlsx) that has been corrupted, entirely. When I open it using Notepad++ it’s just full of NUL bytes. Compare that to a working spread sheet that begins:
PK ! iHëëä â [Content_Types].xml
You can open a normal spread sheet with an archive viewer such as 7-zip, the corrupted one you cannot.
What environment variables can I set to help? Or what logs can I provide to narrow down the issue?