"The file format or file extension is not valid"
The file is called .xlsx, but Excel does not find a workbook in it. It does not offer to open it anyway: it refuses.
What the message says
An .xlsx file is a ZIP archive of XML files. If the start of the file is not an archive, or the archive does not contain a workbook, Excel stops.
Why it appears
- A file of another type renamed to .xlsx (a CSV, an .xls, a web page).
- An empty or truncated file, from an interrupted download or sync.
- A password-protected workbook renamed, or opened by software that cannot decrypt it.
Making it go away
- Look at the file size: at 0 bytes, it is empty and nothing can be recovered.
- Try changing the extension (.xls, .csv) if you know where the file comes from.
- Have the file read by a tool that looks at its content rather than its name.
The tool that handles it
Recover a damaged file says what the file really contains and saves its data; Convert a spreadsheet reads a misnamed CSV or workbook and produces a real XLSX.
Frequently asked questions
- What if the file is password-protected?
- Excel asks for the password when opening. If it shows this message instead, the file has probably been renamed or damaged.