"The file format and extension don't match"
The file is called .xls, but it is not an Excel workbook: it is often a CSV, tab-separated text or an HTML page that some software named that way so Excel would open it.
What the message says
Excel read the start of the file and did not find what the extension promises. It offers to open it anyway, at your own risk.
Why it appears
- A software export (accounting, CRM, website) written as CSV or HTML and named .xls.
- A file renamed by hand.
- More rarely, a damaged or malicious file: do not open a file received from a stranger.
Making it go away
- If the file comes from trusted software: open it, then save it in the real XLSX format.
- Rename a CSV to .csv so Excel opens it as such, separator included.
- Convert the file into a real workbook, accents and numbers fixed.
The tool that handles it
Convert a spreadsheet reads the file from its content, not its extension, and produces a real XLSX. For a CSV with broken accents or everything in one column, Fix a CSV.
Frequently asked questions
- Is it dangerous to open the file anyway?
- Not if it comes from software you use. If it comes from an unexpected email, do not open it.