@Product Are we certain that XLS file types can't be uploaded to Quickbooks Online?
System Error: Upload file attachment in Quickbooks Online caused an error: The file type provided, xls, is not supported by QuickBooks Online Attachments. Supported file types are: PDF, JPEG, PNG, DOC, XLSX, CSV, TIFF, GIF, XML.. Job 03fc7836-3562-4c1f-b34a-b8fa7151733a
I was able to upload this file manually in our test environment without issue so I'm doubting if this is an actual limitation on the API.