Uploading .xlsx file for further processing

Hi everyone,
Is there currently a way to upload an .xlsx file to the OpenAI API?
I’m working with a backend on ASP.NET Core, and no matter what I try, I keep getting a response saying that the file type is not supported.

However, if I use the regular Chat interface in the browser, I can easily upload an .xlsx file without issues.

Could someone please let me know if you’ve managed to get this working?