Can't open CSV file

When I tried to open an exported measurement data in a spreadsheet, Excel can’t recognize the file with csv extension. For example, I have an exported data named “Nuclei.csv” but Excel only read it as “Nuclei.cs” and tells me it can’t be opened.
Could you help me why this happens? Thanks.

Could you post the offending file to this forum thread, so we could take a look?