Hi,
This usually happens when the file content is copied, but the file name doesn't include the original extension.
Here are a few things to check:
In the Create file action (OneDrive), make sure the File Name includes the extension. Instead of entering the name manually, use the File name with extension dynamic content from the SharePoint trigger or Get file metadata action.
If you're using Get file content, remember that it only returns the file content, not the file name or extension. You'll need to pass the file name separately.
If you're copying multiple files, verify that each iteration is using the correct file name and content from the current item.
If the source files are .xlsx and .docx, the created files should have names like:
Report.xlsx
Invoice.xlsx
Letter.docx
If the extensions are missing, Windows will treat them as unknown file types even though the content is correct.
If you're still having issues, could you share a screenshot of the actions you're using (especially Get file content and Create file) and how you've populated the File Name field? That will help identify the exact cause.