"This is an older Office file (.doc, .xls, or .ppt)"

Files ending in .doc and .xls use a closed binary format from before 2007. Every current office app opens them and can save them in the open .docx and .xlsx formats that Okaysize reads.

What to try

Open the file in Microsoft Word or Excel and choose File, Save As, then Word Document (.docx) or Excel Workbook (.xlsx). Google Docs, Google Sheets, Pages, Numbers, and LibreOffice can do the same with Download or Export. Then add the new file here.