GPT being lazy with placeholders

The new GPT4o model is close to working. The Assistant API does seem better but I cannot add an .xlsx document to the file search. I get this error: “Could not attach files to vector store: Files with extensions [.xlsx] are not supported for retrieval”

Do you have any idea how I can use the API to pull a .xslx template and then populate it and return the updated document to the user as a downloadable link?